Add unittests for KisResourceLocator::importResource

The requirements the tests check are the following:

1) When 'override' flag is not set we will never succeed importing,
   even when the existing resource is exactly the same. It is needed
   to avoid confusion in the models that expect the resource to be
   **added** for real.

2) When override flag is set and the exactly the same resource (with the
   same filename case, and with the latest version) exists, then we return
   this resource.

3) Otherwise we remove all the versions of the existing resource and
   add the new one.

CCBUG:446588
CCBUG:446743