Deprecate MediaWikiTestCase::stashMwGlobals

Deprecate MediaWikiTestCase::stashMwGlobals

This method encourages directly editing configuration variables.  It's a
better idea to use setMwGlobals() (or other set wrappers) so that we can
be intelligent in the future, for instance resetting services after the
config change.  Plus, a lot of the callers come out cleaner this way
anyway.

Depends-On: I8a1e81acc5c42a8d7f30938a72cface0acea4a70
Depends-On: I4105dbcf9c5399fe7239478c460ec57c015a98d4
Depends-On: I1b220996acf2f66cf7b0f092b341584663df32f9
Depends-On: Ie2d1ea65c0cb334bbde1666d00781474b7ac4dab
Change-Id: I23d77398e401f4986b1d5bd1c9e11a8a40da16f8