Unify imports of namespace-less classes

Optimizing imports in PHPStorm (Ctrl+Alt+O) removes the line
"use MediaWikiTestCase" for some reason. This may be a bug in
PHPStorm. But this doesn't matter. It seems so common that all the
tests we have use the syntax "extends \MediaWikiTestCase" to avoid
this problem. Except for these few tests. Which I made equal now.

Change-Id: I097cbe306156282768fcaa559f7d55628436a6ee
Merge "Unify imports of namespace-less classes"