Attempt to test --use-feature in pip freeze

I can't get the functional tests to run locally...

Author: rouge8

tests/functional/test_freeze.py

@@ -548,6 +548,7 @@ def test_freeze_nested_vcs(script, outer_vcs, inner_vcs):
     --extra-index-url http://ignore
     --find-links http://ignore
     --index-url http://ignore
+    --use-feature 2020-resolver
 """)