[stubtest] Verify __all__ exists in stub (#18005)

Previously it wasn't an error if runtime included an
`__all__` declaration, but the stubs did not. This PR
changes this to reflect the consensus that it would
be a good idea to ensure consistency in this case.

Fixes #13300

mypy/stubtest.py

@@ -348,6 +348,8 @@ def verify_mypyfile(
             # Only verify the contents of the stub's __all__
             # if the stub actually defines __all__
             yield from _verify_exported_names(object_path, stub, runtime_all_as_set)
+        else:
+            yield Error(object_path + ["__all__"], "is not present in stub", MISSING, runtime)
     else:
         runtime_all_as_set = None
 

mypy/test/teststubtest.py

@@ -1403,7 +1403,7 @@ def test_all_at_runtime_not_stub(self) -> Iterator[Case]:
             runtime="""
             __all__ = []
             Z = 5""",
-            error=None,
+            error="__all__",
         )
 
     @collect_cases
@@ -1443,7 +1443,7 @@ def h(x: str): ...
             runtime="",
             error="h",
         )
-        yield Case(stub="", runtime="__all__ = []", error=None)  # dummy case
+        yield Case(stub="", runtime="__all__ = []", error="__all__")  # dummy case
         yield Case(stub="", runtime="__all__ += ['y']\ny = 5", error="y")
         yield Case(stub="", runtime="__all__ += ['g']\ndef g(): pass", error="g")
         # Here we should only check that runtime has B, since the stub explicitly re-exports it