Add @covers annotations to the tests

[oliverklee]

These make sure that only the lines of the classes the testcases are
about are marked as covered, hence reducing false coverage.

[oliverklee]

Note: This change will reduce the measured code coverage score. On the other hand, the new number will reflect reality much better.

[oliverklee]

Review note: I'll appreciate a critical eye on whether the annotations are complete, or whether we need to add annotations for more covered classes to the test cases.

[sabberworm]

Review note: I'll appreciate a critical eye on whether the annotations are complete, or whether we need to add annotations for more covered classes to the test cases.

I think the ones you marked as covering \Sabberworm\CSS\Parser are also supposed to cover all of the static parse methods like Color::parse, CSSString::parse, etc.

[oliverklee]

@sabberworm Thanks! I will update the annotations accordingly (and rebase).

[oliverklee]

Rebased, updated according to the review comments, and repushed.

[sabberworm]

Rebased, updated according to the review comments, and repushed.

Thanks