Move test requirements to requirements_test.txt

[dokterbob]

Currently, webtest and django-webtest are present in the normal requirements because, by default, Django used to run tests for all pluggable apps. However, I do not think this is currently still the case and perhaps these should be moved.

@claudep What do you think about it?

[claudep]

I'm not familiar with webtest. If we can replace self.app with self.client with no harm, then one less requirement is always better.

[claudep]

Oh, your question was about moving, not removing. So the answer is "sure", of course.