tor-rtmock seems to dislike calling MockRuntime::block_on from multiple threads

Repro steps:

• Check out wesleyac/arti@a6174c36
• cargo test --all-features publish::test::publish_after_ipt_change_with_errors

test tor_hsservice::publish::test::publish_after_ipt_change_with_errors: TEST_TEMP_RETAIN not enabled, using ephemeral temp dir
  Using RNG seed ARTI_TEST_PRNG=ea8a521c1cde2bba120feb4fdf58a3a59418b38bdd38dbe772b0d3f40c159c30
thread '<unnamed>' panicked at crates/tor-rtmock/src/task.rs:480:17:
assertion `left == right` failed: ProgressingUntilStalled finished twice?!
  left: Ready(())
 right: Pending