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
testPngNativetZtxt requires zlib extension

Change-Id: I27e84548ff792ee3bac9b582ba058a029a786768
test: some Revision tests depends on PHP zlib ext

Use an helper to verify whether 'zlib' is available, else will happilly
skip the test.

Change-Id: I43f8b20a95286b5495799d54b415f6d15894a335
Merge "testPngNativetZtxt requires zlib extension"