qunit: Make use of 'config' and 'messages' shortcuts in module setup

While calling mw.config and mw.messages manually works fine (the sandbox
covers mw.config and mw.messages in general, not just when using the
shortcut), let's consistently use the shortcuts.

Also clean up the extra local variable in some cases.

Change-Id: I9ac467ad87816c7dc681b62e9b085b643024c15e
Merge "qunit: Make use of 'config' and 'messages' shortcuts in module setup"