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

crates/gpui/Cargo.toml

@@ -4,6 +4,7 @@ version = "0.1.0"
 edition.workspace = true
 authors = ["Nathan Sobo <nathan@zed.dev>"]
 description = "Zed's GPU-accelerated UI framework"
+repository = "https://github.com/zed-industries/zed"
 publish.workspace = true
 license = "Apache-2.0"