In the class MediaWikiTestCase, there is one test automatically added, testMediaWikiTestCaseParentSetupCalled, checking that the setUp function calls its parent. This test does not have @covers or @uses annotations.
When I tested my extension MediaWikiFarm with test classes inhereting from MediaWikiTestCase, this test is marked as "risky" by PHPUnit 4.8 when the strict code coverage is activated (--coverage-html docs/coverage --strict-coverage). I modified the MediaWiki file tests/phpunit/suite.xml to whitelist my extension code, and removed MediaWiki internals (directories includes, etc.) to speedup the code coverage, so perhaps it does not like it, but I just checked with the very standard version of MediaWiki without extension and the test is still reported as "risky" because a zillion of functions are called (in setUp, etc.) but not marked as @covers or @uses.
To avoid marking this test risky because of absence of @covers, there would be the possibility of adding zillions of @covers but this would be a bad choice because any extension extending MediaWikiTestCase would still trigger the non-@covers warning. Instead, the test could be annotated by @coversNothing. I tested and the test now pass without warning. This would mark as "pass" instead of "risky" as much tests there are test classes, currently 478 in core.