test: helper to skip tests depending on a PHP ext

test: helper to skip tests depending on a PHP ext

Some of our tests expect a specific PHP extension to be loaded to get
anything done, for example zlib or gd.  This patch creates a new helping
method that people can use to easily skip a test whenever a PHP
extension is not around: MediaWikiTestCase::checkPHPExtension()

Example usage:

 function testCompressFiles() {
   $this->checkPHPExtension( 'zlib' );
   ...
 }

Change-Id: Ia87317ca379b2d5d1d1fa4231f76033ee66086c2