Improve PHPUnit detection code

The code that attempts to load PHPUnit is simpler and clearer when rewritten as
a loop that iterates through a set of possible include paths for PHPUnit,
ordered from most to least desirable, and which tries each include path in turn
and then checks if the requisite classes are now loaded.

Also simplify check for PHP version: MediaWiki requires 5.3+, so might as well
assume it.

Change-Id: I9e25d69c1381cf3a87e7df2baf346bc6bb5aa052