Add new TestCaseCovers sniff to control @covers tags in unit tests

[stronk7]

This new feature will be in charge of validating the presence and
correctness of PHPUnit coverage annotations/tags.

Restrictions:

• It's only applied to Moodle 4.0 and up.
• It's only applied to files under /tests directories.
• It's only applied to files named _test.php.
• It's only applied to classes named _test or _testcase.
• it's only applied to methods named test_ .

Warnings implemented:

• Warn if both @covers and @coversNothing information are missing.
• Warn when there are contradictory mixed @coversNothing (at class) and
  @covers (at method) combinations.
• Warn about redundant @coversNothing present both at class and method.

Errors implemented:

• Error when there are contradictory @coversNothing and @covers in the
  same class or method.
• Error if the @coversDefaultClass is used in method.
• Error if the @coversNothing tag has any value.
• Error if the @covers or @coversDefaultClass tags don't have any value.
• Error if the @coversDefaultClass tag doesn't start with \ (FQCN).
• Error if the @coversDefaultClass tag has :: (reserved for @covers).
• Error if the @covers tag doesn't start with \ (FQCN) or :: (method).

96.67% line coverage achieved (soon will be 100%, another PR incoming).

Impact notes: See that, while adding @covers annotations to various
unit tests, it's also mandatory to add a description to them. That's
because the local_moodlecheck (in charge of examining phpdoc blocks)
accepts the absence of phpdoc blocks, but once they exist, the
description in mandatory. Not sure if we should relax that in
moodlecheck or no (being strict, I think it's correct).

[stronk7]

For the records, have run it against core with:

$ local/codechecker/phpcs/bin/phpcs --standard=moodle  --extensions=php --sniffs=moodle.PHPUnit.TestCaseCovers .

And:

• run took 8 minutes.
• 9329 warnings (99% of them about missing annotations.
• 3 errors.

Ciao :-)

[stronk7]

Going ahead, after discussing it with the team (also was shared some days ago @ dev chat, just in case).