Add missing raises keyword for test_depth xfail

I had forgotten to do this earlier when converting from skip to
xfail. Besides consistency with the other uses of xfail in the test
suite, the benefit of passing "raises" is that pytest checks that
the failure gave the expected exception and makes it a non-expected
failure if it didn't.

test/test_submodule.py

@@ -1049,6 +1049,7 @@ class Repo(object):
 
     @pytest.mark.xfail(
         reason="for some unknown reason the assertion fails, even though it in fact is working in more common setup",
+        raises=AssertionError,
     )
     @with_rw_directory
     def test_depth(self, rwdir):