Handle phpunit being autoloaded from checkLess.php

When phpunit is available via the autoloader, rather than directly in the
include path, checkLess was erroring out.  Additionally
RequestContext::resetMain() was outputting warnings due to MW_PHPUNIT_TEST
not being defined.

Change-Id: I28921c018daa59b2da78ffb5312690d1ce8fbb0a
Merge "Handle phpunit being autoloaded from checkLess.php"