tests: Fix valid_tags mocking
tests: Fix valid_tags mocking

The "test_resolving_respects_platform" test was broken on Pip 19.1,
because the test mocks "finder.valid_tags", but it is moved to
"finder.candidate_evaluator._valid_tags" in Pip 19.1.  Fix the mocking
so that the test works for Pip 19.1 too.

suutari-ai/prequ,suutari/prequ,suutari/prequ