Reset $wgContLang consistently for tests

Reset $wgContLang consistently for tests

We already reset $wgParser to a stub in a number of places where it
might have changed. Let's give $wgContLang the same treatment so we can
avoid special-case code.

This deprecates setContentLang(), which is no longer needed. The magic
of $wgContLang is now handled by setMwGlobals() and setService().

This is a follow-up to e4f69ee, which fixed one case of this problem.

Change-Id: I90925ef8b2a7478cce90d474db1b8b4539e45c15