tor-circmgr test "request_retried" is not reliable

At https://gitlab.torproject.org/nickm/arti/-/jobs/45855 I see:

...
thread 'mgr::test::request_retried' panicked at 'called `Result::unwrap()` on an `Err` value: RequestFailed(RetryError { doing: "find or build a circuit", errors: [], n_errors: 0 })', crates/tor-circmgr/src/mgr.rs:1575:25
advancing released: circuit builder task 6132487541863617059
failures:
    mgr::test::request_retried

I couldn't reproduce this just by running this test on its own, but I managed to get it to reoccur by running all of the tor-circmgr tests in a loop:

; while cargo test -p tor-circmgr  ; do : ; done
...
sleeper made for 59.97s, 64/65
sleeper dropped, incrementing count
sleeper polled, 65/65
setting advance flag
waitfor done!
thread 'mgr::test::request_retried' panicked at 'called `Result::unwrap()` on an `Err` value: RequestFailed(RetryError { doing: "find or build a circuit", errors: [], n_errors: 0 })', crates/tor-circmgr/src/mgr.rs:1575:25