Automatically reset namespace caches when needed

Automatically reset namespace caches when needed

This avoids error-prone code written separately in every test.  In
addition to no existing tests resetting the TitleFormatter (more
services probably need to be reset as well), they mostly reset only the
namespace cache on $wgContLang, which wouldn't help for any other
language.

The parser test runner still doesn't do this, but maybe it should.

Change-Id: I44b7a1aec48f14b0950907fa14bd0df80f674296