Add `seed` argument to `#[gpui::test]` attribute macro (#26764)

This PR introduces the arguments `seed` and `seeds` to `gpui::test`,
e.g.:
- `#[gpui::test(seed = 10)]`
- `#[gpui::test(seeds(10, 20, 30, 40))]`

Which allows us to run a test against a specific seed value without
slowing
down our tests like `iterations` does with high values.

This was motivated by a diff hunk test that only fails in a 400+ seed,
but is
slow to run 400+ times for every `cargo test`.

If your test failed with a specific seed, you can now add the `seed` arg
to
increase the chances of detecting a regression.

There are now three ways of setting seeds, the `SEED` env var,
`iterations`,
and the args this PR adds. See docs in `gpui::test`.

---

I also relaxed the limitation on `retries` not working with
`iterations`, as
that seemed unnecessary.

Release Notes:

- N/A