chore: fix compilation on master

[Darksonn]

Master fails with the following warning on wasm:

error: unused import: `wasm_bindgen_test::wasm_bindgen_test as test`
 --> tokio/tests/sync_mpsc_weak.rs:6:5
  |
6 | use wasm_bindgen_test::wasm_bindgen_test as test;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `-D unused-imports` implied by `-D warnings`

This is due to a PR that got merged by auto-merge even though it doesn't pass CI.

I fixed this by using #[test] for the non-async tests.