python: warn when an abstract test class swallows its tests

[RonnyPfannschmidt]

Written by Claude Opus 5 via Claude Code for the pytest maintainers; I prompted it, it did the work, I read it.

Draft, for the design question in #13546 rather than for merge as it stands.

Abstract classes have not been collected since #12318, which fixed the Can't instantiate abstract class regression of #12275. That is right for a base class written to be subclassed, but a leaf class that merely forgot to implement a blank looks exactly the same, so its tests stop running and nothing says so. Reported four times: #13546, #14003, #15080, #15081.

This warns when an abstract class matches the test-class naming conventions (or is a unittest.TestCase subclass), carries tests, declares no abstract method of its own, and does not set __test__ in its own body. Declaring a blank is read as meaning to be a base class, so the template-base pattern stays silent and no new API is needed. The __test__ opt-out is read off the class rather than through inheritance, because an inherited __test__ would silence every subclass too.

The unittest hook warns for itself and returns a result rather than None, both because unittest classes are collected regardless of python_classes — aiohttp's InvalidTestCase in #13546 would otherwise be missed — and to keep the python plugin from warning about the same class a second time.

The open question, and why this is a draft: TestPartial in test_abstract_class_is_not_collected is a deliberate intermediate layer that implements one blank and adds none, so it now warns. This PR makes that test ignore the warning rather than paper over it. A deliberate middle layer and a leaf that went dark are textually identical; suppressing the false positive also suppresses the "base class grew a new abstract method" case that motivates the whole change. __test__ = False in the body is the escape hatch. See #13546 for the reasoning.

Not included: @abstractmethod on a class without ABCMeta is collected, called, and passes vacuously. Separate failure mode, separate issue.

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