[devtools/testlib] Remove assertions about direct usage of CubicWebTC/AutomaticWebTest
The point is to be able to use a test runner other than pytest (e.g. nose2,
nosetests or py.test) to run CubicWeb test suite. The issue is that most other
test loaders inspects members of all base classes of test case classes to
determine test methods and it hits this `config` property (actually executing
it), which breaks the assertion. So just return None instead of failing.
Closes #7817147.
