Improve namespace handling in tests

Improve namespace handling in tests

MWNamespace has three internal caches, only one of which can be cleared
(and that somewhat oddly by passing a boolean to
MWNamespace::getCanonicalNamespaces()).

This change introduces a MWNamespace::clearCaches() method to clear all
three caches. It also adds some resetting in tests that seemed to be
missing it.

Change-Id: I1dcfcd8713888b3ff8fc75e95329ba72bd95d0c9