tor-circmgr spurious test failure

In https://gitlab.torproject.org/Diziet/arti/-/jobs/400292,

test build::test::build_modest_timeout has been running for over 60 seconds
ERROR: Job failed: execution took longer than 1h0m0s seconds

The source code for this test, and others, has BUG comments.

See also #919.

When we have a spare half week we could rewrite tor-circmgr tests to use new MockRuntime which ought to make all the tests deterministic.