In MediaWikiTestCase::stashMwGlobals(), prefer shallow copies

In MediaWikiTestCase::stashMwGlobals(), prefer shallow copies

MediaWikiTestCase::stashMwGlobals() performs a deep copy of globals by
serializing and then unserializing them. This is actually unwarranted in the
common case of simple scalar values and flat arrays -- and it's expensive, too.
So before attempting a deep copy, first check if a shallow copy will do.

Change-Id: Iaba1c8e1f6bae9de0a7a1fb411cac94f7e4dfb23