watch: fix spurious wakeup

[Darksonn]

Closes: #3168

I will push the fix that @tijsvd suggested once I have observed the test failing on CI. It does fix the test locally.

loop {
        let notified = self.shared.notify_rx.notified();
        if let Some(ret) = maybe_changed(&self.shared, &mut self.version) {
            return ret;
        }
        notified.await;
}

command to run the test:

LOOM_MAX_PREEMPTIONS=1 RUSTFLAGS="--cfg loom" cargo test --lib --release --features full -- --test-threads=1 --nocapture watch_spurious_wakeup

[Darksonn]

The test failure has been observed in CI / loom (--skip loom_pool) (pull_request).

thread 'sync::watch::tests::watch_spurious_wakeup' panicked at '[bug] failed to observe change after notificaton.', tokio/src/sync/watch.rs:256:14
stack backtrace:
   0: rust_begin_unwind
             at /rustc/7eac88abb2e57e752f3302f02be5f3ce3d7adfb4/library/std/src/panicking.rs:483
   1: core::panicking::panic_fmt
             at /rustc/7eac88abb2e57e752f3302f02be5f3ce3d7adfb4/library/core/src/panicking.rs:85
   2: core::option::expect_failed
             at /rustc/7eac88abb2e57e752f3302f02be5f3ce3d7adfb4/library/core/src/option.rs:1226
   3: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
   4: futures_util::future::future::FutureExt::now_or_never
   5: core::ops::function::FnOnce::call_once{{vtable.shim}}
   6: loom::rt::scheduler::spawn_threads::{{closure}}::{{closure}}
   7: generator::stack::StackBox<F>::call_once
   8: generator::gen_impl::gen_init
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
test sync::watch::tests::watch_spurious_wakeup ... FAILED
Completed in 123 iterations