Introduce stashMwGlobals method to MediaWikiTestCase

This method is factored out from the existing
seMwGlobals method.

The Doc from the initial method is also split and
improved and since tags has been added

Also adds tests

Change-Id: I0637194d637abf485a245b00587743f0f6dd495a
Throw exception when trying to stash unset globals

If we are trying to stash an unset global then
throw an exception!

Also adds a test

Change-Id: I25f493a0a535201c08ca9624c2c650f61d9e256d
Merge "Introduce stashMwGlobals method to MediaWikiTestCase"
Merge "Throw exception when trying to stash unset globals"