Provide PHPUnit 4 and 6 compatibility layer

Provide PHPUnit 4 and 6 compatibility layer

PHPUnit 6 removed some functions that were heavily used in version 4. To
be able to support both versions for a short time, we'll use a trait to
fill in the missing methods until we drop PHPUnit 4 support.

This trait is included in MediaWikiTestCase so most tests will be able
to benefit from it by default. Otherwise, anything that calls
setExpectedException() or getMock() will need to use it.

Change-Id: I707129e471e960e034e2aa994a467b9dc0239b69