Add tests for unions derived from abstract unions

Summary:
... and remove unnecessary tests to reduce verbosity.

# Rationale
{D66151439} above this one changes the implementation of unions in the presence of abstract types and there were no tests that would catch regression. Introduce the test first to ensure it works correctly with the current implementation. It will then serve as the safety net when D66151439 makes its change.

# Action Items
Generate fixtures with
```
buck2 run fbcode//thrift/compiler/test:build_fixtures
```

Reviewed By: ahilger

Differential Revision: D66214545

fbshipit-source-id: f58ce5811bfc708c774e97021f10ff8b26026350

thrift/test/thrift-python/abstract_types_test.py

@@ -87,7 +87,9 @@ def setUp(self) -> None:
         # Disable maximum printed diff length.
         self.maxDiff = None
 
-    def test_fn_call_with_read_only_abstract_base_class_with_immutable(self) -> None:
+    def test_fn_call_with_read_only_abstract_base_class_with_immutable_struct(
+        self,
+    ) -> None:
         """
         type-check will fail with the error below if
         TestStructImmutable does not inherit from TestStructAbstract.
@@ -98,48 +100,15 @@ def test_fn_call_with_read_only_abstract_base_class_with_immutable(self) -> None
         # GIVEN
         def library_fn(ts: TestStructAbstract) -> None:
             # THEN
-            self.assertEqual(ts.unqualified_string, "hello")
-            self.assertIsNone(ts.optional_string)
+            self.assertIsInstance(ts, TestStructAbstract)
 
         # WHEN
         library_fn(TestStructImmutable(unqualified_string="hello"))
-
-    def test_type_hint_with_read_only_abstract_base_class_with_immutable(self) -> None:
-        """
-        type-check will fail with the error below if
-        TestStructImmutable does not inherit from TestStructAbstract.
-
-        Incompatible variable type [9]: ts is declared to have type `TestStructAbstract` but is used as type `TestStructImmutable`.
-        """
-        # GIVEN
-        # TestStructAbstract
-        # TestStructImmutable
-
-        # WHEN
-        ts: TestStructAbstract = TestStructImmutable(unqualified_string="hello")
-
-        # THEN
-        self.assertEqual(ts.unqualified_string, "hello")
-        self.assertIsNone(ts.optional_string)
-
-    def test_isinstance_with_read_only_abstract_base_class_with_immutable(self) -> None:
-        """ """
-        # GIVEN
-        # TestStructAbstract
-        # TestStructImmutable
-
-        # WHEN
-        # This assignment is here to validate type-checking.
-        ts: TestStructAbstract = TestStructImmutable(unqualified_string="hello")
-
-        # THEN
-        self.assertIsInstance(ts, TestStructAbstract)
-
-    def test_issubclass_with_read_only_abstract_base_class_with_immutable(self) -> None:
-        """ """
         self.assertTrue(issubclass(TestStructImmutable, TestStructAbstract))
 
-    def test_fn_call_with_read_only_abstract_base_class_with_mutable(self) -> None:
+    def test_fn_call_with_read_only_abstract_base_class_with_mutable_struct(
+        self,
+    ) -> None:
         """
         type-check will fail with the error below if
         TestStructMutable does not inherit from TestStructAbstract.
@@ -150,45 +119,49 @@ def test_fn_call_with_read_only_abstract_base_class_with_mutable(self) -> None:
         # GIVEN
         def library_fn(ts: TestStructAbstract) -> None:
             # THEN
-            self.assertEqual(ts.unqualified_string, "hello")
-            self.assertIsNone(ts.optional_string)
+            self.assertIsInstance(ts, TestStructAbstract)
 
         # WHEN
         library_fn(TestStructMutable(unqualified_string="hello"))
+        self.assertTrue(issubclass(TestStructMutable, TestStructAbstract))
 
-    def test_type_hint_with_read_only_abstract_base_class_with_mutable(self) -> None:
+    def test_fn_call_with_read_only_abstract_base_class_with_immutable_union(
+        self,
+    ) -> None:
         """
         type-check will fail with the error below if
-        TestStructMutable does not inherit from TestStructAbstract.
+        TestUnionImmutable does not inherit from TestUnionAbstract.
 
-        Incompatible variable type [9]: ts is declared to have type `TestStructAbstract` but is used as type `TestStructMutable`.
+        Incompatible parameter type [6]: In call `ThriftPythonAbstractTypesTest.test_fn_call_with_read_only_abstract_base_class_with_immutable.library_fn`, for 1st positional argument, expected `TestUnionAbstract` but got `TestUnionImmutable`.
         """
+
         # GIVEN
-        # TestStructAbstract
-        # TestStructMutable
+        def library_fn(ts: TestUnionAbstract) -> None:
+            # THEN
+            self.assertIsInstance(ts, TestUnionAbstract)
 
         # WHEN
-        ts: TestStructAbstract = TestStructMutable(unqualified_string="hello")
+        library_fn(TestUnionImmutable(string_field="hello"))
+        self.assertTrue(issubclass(TestUnionImmutable, TestUnionAbstract))
 
-        # THEN
-        self.assertEqual(ts.unqualified_string, "hello")
-        self.assertIsNone(ts.optional_string)
+    def test_fn_call_with_read_only_abstract_base_class_with_mutable_union(
+        self,
+    ) -> None:
+        """
+        type-check will fail with the error below if
+        TestUnionMutable does not inherit from TestUnionAbstract.
+
+        Incompatible parameter type [6]: In call `ThriftPythonAbstractTypesTest.test_fn_call_with_read_only_abstract_base_class_with_mutable.library_fn`, for 1st positional argument, expected `TestUnionAbstract` but got `TestUnionMutable`.
+        """
 
-    def test_isinstance_with_read_only_abstract_base_class_with_mutable(self) -> None:
-        """ """
         # GIVEN
-        # TestStructAbstract
-        # TestStructMutable
+        def library_fn(ts: TestUnionAbstract) -> None:
+            # THEN
+            self.assertIsInstance(ts, TestUnionAbstract)
 
         # WHEN
-        ts: TestStructAbstract = TestStructMutable(unqualified_string="hello")
-
-        # THEN
-        self.assertIsInstance(ts, TestStructAbstract)
-
-    def test_issubclass_with_read_only_abstract_base_class_with_mutable(self) -> None:
-        """ """
-        self.assertTrue(issubclass(TestStructMutable, TestStructAbstract))
+        library_fn(TestUnionMutable(string_field="hello"))
+        self.assertTrue(issubclass(TestUnionMutable, TestUnionAbstract))
 
     @parameterized.expand(
         [