Test case "thread ' scheduler::test::it_cancels_delayed_firings" is flaky

Spurious failure here: https://gitlab.torproject.org/Diziet/arti/-/jobs/168264

failures:
---- scheduler::test::it_cancels_delayed_firings stdout ----
thread 'scheduler::test::it_cancels_delayed_firings' panicked at 'assertion failed: sch.next().now_or_never().is_none()', crates/tor-rtcompat/src/scheduler.rs:334:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
failures:
    scheduler::test::it_cancels_delayed_firings
test result: FAILED. 44 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.93s