no cover for xfail'ed test_forward_reference()

`test_forward_reference()` is unconditionally xfail'ed and as such we don't expect it to get fully covered.  May as well not include it in coverage results then.

Author: altendky

tests/test_make.py

@@ -378,7 +378,7 @@ class A:
         + "See https://github.com/lovasoa/marshmallow_dataclass/issues/13"
     ),
 )
-def test_forward_reference(module, assert_dump_load):
+def test_forward_reference(module, assert_dump_load):   # pragma: no cover
     """Build schemas from classes that are defined below their containing class."""
 
     @module.dataclass