MWDebugTest: disable MWDebug on test teardown

MWDebugTest: disable MWDebug on test teardown

MWDebug::init() is currently irreversible -- once MWDebug is enabled, it cannot
be disabled in that execution context. This means that the MWDebug test suite
(which enables MWDebug) has a nasty side-effect -- all the tests that run after
it run with MWDebug enabled. So add an MWDebug::deinit(), and call it on test
teardown.

Ostensibly this is a great use-case for services and dependency injection. The
reason I am not going that route is that it's not entirely clear to me what the
MWDebug class is supposed to represent. If I were going to spend any
substantial amount of time on this, I would be trying to move it out of core
and into an extension, not converting it into a service.

Change-Id: I52c511be049bc276d203d07283e3aa0944f22d34