Add test coverage for test selection insights

[novemberborn]

As of https://github.com/avajs/ava/pull/2841 AVA prints warnings when file paths were provided on the CLI that could not be used to select test files, or otherwise when the CLI arguments result in zero test files being selected. There's no test coverage for this output yet.

Generally speaking we don't have an easy way to test the reporter output. If you're keen to tackle this please leave a comment. Otherwise this is just a tracking issue.