disable test_warnings

it is impossible to determine the color given the output text alone

Author: asottile

tests/pygments_pytest_test.py

@@ -82,6 +82,7 @@ def test_simple_test_passing(compare):
     compare('def test(): pass')
 
 
+@pytest.mark.xfail(reason='no output for a warning but the [...%] is yellow')
 def test_warnings(compare):
     compare(
         'import warnings\n'