Make parity-failure output more obvious

Print each offender on its own line, labelled with which side is missing
the counterpart, instead of dumping two summary sentences with embedded
list reprs. Factor the duplicated set-comprehension into a small inner
helper so the assertion body is also a touch shorter.

Sample failure (deleting one mirrored test, adding two extras):

    TestPEP828Operation is not a 1:1 mirror of TestPEP380Operation (suffix '_ayf'):
        missing in TestPEP828Operation: test_yield_from_empty_ayf
        no counterpart in TestPEP380Operation: test_made_up_extra_ayf
        no counterpart in TestPEP380Operation: test_other_extra_ayf

Author: johnslavik

Lib/test/test_async_yield_from.py

@@ -1630,29 +1630,24 @@ def assert_parity(self, base_class, variant_class, *, suffix):
         ``test_xxx<suffix>`` on ``variant_class`` and vice versa. Variant-only
         tests belong in a separate TestCase class.
         """
-        base_tests = {
-            n for n in dir(base_class)
-            if n.startswith("test_") and callable(getattr(base_class, n))
-        }
-        variant_tests = {
-            n for n in dir(variant_class)
-            if n.startswith("test_") and callable(getattr(variant_class, n))
-        }
-        expected = {n + suffix for n in base_tests}
-        missing = sorted(expected - variant_tests)
-        extra = sorted(variant_tests - expected)
-        problems = []
-        if missing:
-            problems.append(
-                f"{variant_class.__name__} missing variants of "
-                f"{base_class.__name__} tests (suffix {suffix!r}): {missing}"
-            )
-        if extra:
-            problems.append(
-                f"{variant_class.__name__} has tests with no counterpart in "
-                f"{base_class.__name__} (suffix {suffix!r}): {extra}"
-            )
-        self.assertEqual(problems, [], "\n".join(problems))
+        def test_methods(cls):
+            return {n for n in dir(cls)
+                    if n.startswith("test_") and callable(getattr(cls, n))}
+
+        expected = {n + suffix for n in test_methods(base_class)}
+        actual = test_methods(variant_class)
+        missing = sorted(expected - actual)
+        extra = sorted(actual - expected)
+        if missing or extra:
+            lines = [
+                f"{variant_class.__name__} is not a 1:1 mirror of "
+                f"{base_class.__name__} (suffix {suffix!r}):"
+            ]
+            for name in missing:
+                lines.append(f"    missing in {variant_class.__name__}: {name}")
+            for name in extra:
+                lines.append(f"    no counterpart in {base_class.__name__}: {name}")
+            self.fail("\n".join(lines))
 
     def test_TestPEP828Operation(self):
         self.assert_parity(