Verify that all @covers tags are sane when running tests

Verify that all @covers tags are sane when running tests

PHPUnit only verifies that @covers tags are correct when actually
running coverage - but that's too slow to do on every commit. And if
even a single tag is incorrect, the entire coverage job will fail. It
also has some different requirements compared to normal tags, like all
namespaced classes must use their absolute name. All of those things
combined make it easy for developers to accidentally break the coverage
job.

There are some external tools that also do this, but those tools are
incompatible with our unusual PHPUnit setup (the phpunit.php wrapper
script).

The MediaWikiCoversValidator trait just calls the same method that
PHPUnit does to validate @covers tags. It is implemented as a trait so
that test cases that implement PHPUnit_Framework_TestCase directly can
still use this.

Bug: T171899
Change-Id: I1d564bcae2bfbedb004c440b90db6341148ed4ba