uv-pep508: add a clarifying test
This test demonstrates the difference between `extra != "foo"` and
`sys_platform != "foo"`.
I wrote this test down to test the extra simplification logic was
correct. And I also wanted to test whether we could somehow hackily
encode `group` (as opposed to just `extra`) logic into marker
expressions by reusing another field. But I don't think we can.
