Make error message for specs without ".abi3." more prescriptive

Say that the file path must contain `.abi3.` and check that this is
indeed the case in the corresponding test.

abi3audit/_extract.py

@@ -104,7 +104,7 @@ def make_specs(val: str) -> list[Spec]:
         # audited (e.g. via an abi3 wheel), but not directly (since
         # without a tag here we don't know if it's abi3 at all).
         if ".abi3." not in val:
-            raise InvalidSpec(f"'{val}' looks like a shared object but is not tagged as abi3")
+            raise InvalidSpec(f"'{val}' must contain '.abi3.' to be recognized as a shared object")
         return [SharedObjectSpec(val)]
     elif re.match(_DISTRIBUTION_NAME_RE, val, re.IGNORECASE):
         return [PyPISpec(val)]

test/test_extract.py

@@ -17,7 +17,7 @@ def test_make_spec():
     assert make_specs("foo") == [PyPISpec("foo")]
 
     # Shared objects need to be tagged with `.abi3`.
-    with pytest.raises(InvalidSpec):
+    with pytest.raises(InvalidSpec, match="'foo.so' must contain '.abi3.'"):
         make_specs("foo.so")
 
     # Anything that doesn't look like a wheel, shared object, or PyPI package fails.