replcompletions: Try to make the test more robust (#59166)
This is an alternative to #59161, attempting to fix the same observed CI
behavior. I don't think #59161 is the best way to fix it, as the point
of these tests is to make sure that REPL completions looks up the PATH
internally. Calling the path update function explicitly defeats that
somewhat. The extra synchronization here to make this deterministic is
messy, but I do think it makes the test closer to real-world usage.
The core attempted fix here is to move the read of the PATH_ locals
inside `maybe_spawn_cache_PATH` into the locked region. If they are not
under the lock, they could be unconditionally overwritten by a second
call to this function, causing issues in the state machine. I do not
know whether this is the cause of the observed CI hangs, but it's worth
fixing anyway.
