sync: fix fuzz_oneshot test by using instrumented `loom::sync::Arc` (#1464)
sync: fix fuzz_oneshot test by using instrumented `loom::sync::Arc` (#1464)

Since `tokio_sync::oneshot` makes a `CausalCell::with_mut()` mutable
access in the `Inner::drop()`, we must use the instrumented
`loom::sync::Arc`.

Uncovered by carllerche/loom#42
tokio-rs/tokio,tokio-rs/tokio