warn about tests returning any non-None value

[graingert]

Currently there's a warning for coroutine functions (async def) but not for

twisted.internet.defer.inlineCallbacks, where the only way to tell is the fact that the test returned a deferred

• a detailed description of the bug or suggestion
• output of pip list from the virtual environment you are using
• pytest and operating system versions
• minimal example if possible

[Zac-HD]

Closing as a duplicate of #7337 - while that issue is considerably newer, it also has the consensus plan to warn (and eventually error) on non-None return values 🙂