Skip universal tests on macOS and Windows (#20252)
## Summary
This PR adds a `test-universal` feature for test cases whose results do
not depend on the host platform. It remains in `test-defaults` and is
explicitly enabled on Linux, but omitted on macOS and Windows (which
continue to use `--no-default-features` without opting in).
The dedicated lock and lock-scenario suites, export and branching URL
tests, universal `pip compile` and `uv tree` cases, and lock-only
workspace cases are all gated under this flag. The sync and install
tests are omitted (i.e., still run on each platform) since they install
platform-specific artifacts. In the macOS CI feature configuration, this
removes 660 tests from the affected binaries.
In [CI on this
branch](https://github.com/astral-sh/uv/actions/runs/28961560957)
compared with [CI for the exact base
commit](https://github.com/astral-sh/uv/actions/runs/28961428604), macOS
nextest wall time fell from 6:29 to 4:31 (30%) and the full `Cargo test`
step fell from 9:42 to 7:03, saving 2:39 (27%). Windows ran 616 fewer
tests; nextest's critical partition fell from 1:26 to 1:17 (10%) and
aggregate test-case time across the three partitions fell by 7:04 (13%),
although compilation and cache variance masked that gain in the full
`Cargo test` step, which was 9 seconds slower in this sample.
