Add another motherless test

This test is for videos posted in groups.
URL looks like this:
motherless.com/g/group_name/ID
which is basically the same as
motherless.com/ID
So far this test fails because I haven't improved the extractor yet.
Add group videos support for motherless

Modified motherless regular expression.
Previously it matched only URLs like this:
motherless.com/ID
Now it also matches this:
motherless.com/g/group_name/ID
All tests including the newly added one passed.