django-compressor served it's purpose, but given the advent of
more and more frontend JS work, and the accompanying build tools (grunt,
gulp, etc.), it makes less and less sense to use compressor. Jannis
Leidel himself also stepped back from maintaining it.

In my experience with Oscar project, it has also become a bit of a
liability to have compressor as a dependency. Most projects don't use it
any more, but it's not easy to disable/override because all our
templates include it.

So, let's keep things simple and remove it as a dependency. That paves
the way for using other tools, while it's always easy to add back.

I'm not sure why @codeinthehole, back in the days,  opted to
manually configure settings instead of using a regular settings file for
the test suite. Most likely the reasons for doing so have disappeared.

So I turned our tests/config.py into a normal settings file, and use
that accordingly. That means we do things the standard Django way, and
it's easier to drop into a session with the test settings.