Correctly add a reason to the xfail

Author: pradyunsg

tests/functional/test_install_reqs.py

@@ -211,7 +211,7 @@ def test_multiple_constraints_files(script, data):
     assert 'installed Upper-1.0' in result.stdout
 
 
-@pytest.mark.xfail("Unclear what this guarantee is for.")
+@pytest.mark.xfail(reason="Unclear what this guarantee is for.")
 def test_respect_order_in_requirements_file(script, data):
     script.scratch_path.joinpath("frameworks-req.txt").write_text(textwrap.dedent("""\
         parent