The automatic test in MediaWikiTestCase was marked as risky

The automatic test in MediaWikiTestCase was marked as risky

MediaWikiTestCase::testMediaWikiTestCaseParentSetupCalled is
marked as risky when strict code coverage is activated in
PHPUnit (--coverage-html ../../docs/code-coverage
--strict-coverage --debug -v).

This patch adds the annotation @coversNothing to disable
code coverage on this test. As an automatically-added test,
it must adapt to a number of different environments, so it
is not feasible to add the exhaustive list of @uses classes.

Bug: T152923
Change-Id: Ic670ff2d89f1c5df500c0d10b153d4efd013d7f4