Use project url for tests deriving from Mezzanine's TestCase and
empty urls otherwise -- that's what django.contrib tests presume.

Use Apps functionality rather than INSTALLED_APPS, as it's unlikely
this runner will be compatible with 1.6 anyway (planning to only
enable it on a version >= 1.7 condition).
Mezzanine tests should use Mezzanine's TestCase.
Switched to double suite building procedure.
With the double suite building approach hard-coded importing of
defaults isn't necessary anymore.
Made the new runner the default only for 1.7+.

Separated django.contrib tests preparation to a contextmanager
to make it clear which workarounds are for what.