fix nested describe.only suites; closes #2406

[not-an-aardvark]

Previously, tests in nested describe.only suites would not be registered as only tests in their parent suite (due to the implementation described in #2387 (comment)). This caused mocha to think that the parent suite had no only tests, so the suite was skipped entirely. (See #2406 for more information.)

This PR moves some of the logic for it.only and describe.only to the test runner, rather than the place where suites are created. This makes the logic slightly simpler, because all of the suites already exist at the time that only suites are handled.

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The correct logic for only blocks (from what I can tell, looking at the existing tests and the docs) is as follows:

To run a suite:

• If a suite contains no only suites/tests among all of its descendants, run all of its child suites and tests.
• Otherwise, if a suite contains any only tests among its children, run the only tests for that suite, and skip all the child suites.
• Otherwise, run all the child only suites. Also run any child suites that have only suites/tests as descendants.

[boneskull]

Thanks for this. I'm re-running the appveyor build; it may have been timing flake.

[boneskull]

At first glance, this looks good. If it passes CI, I'll see if I can't release it as v3.0.1 tonight or tomorrow.

[boneskull]

@not-an-aardvark Is there anything I should know about with the signed commit? I haven't worked with these before, and don't know the implications (if any).

[not-an-aardvark]

I don't think there are any implications to worry about; it basically just means you can be sure that the person with the not-an-aardvark GitHub account is the same person who made the commit. (This information would be more useful if another user had a branch with some of my commits on it, for example. In this case it's sort of trivial because I made the PR anyway.)

[boneskull]

@not-an-aardvark tyvm!