Concurrency bug(s) with watchdogs

[lhstrh]

The issue(s) appears to be limited to macOS, but it might not be: https://github.com/lf-lang/reactor-c/actions/runs/10223945465/job/28292397672

    +---------------------------------------------------------------------------+
    Failed: src/concurrent/Watchdog.lf in 300.04 seconds

    -----------------------------------------------------------------------------
    Reason: Test timed out.

and

...
    DEBUG: There are 1 trigger arrays associated with output 0.
    DEBUG: Trigger 0x144008278 lists 1 reactions.
    DEBUG: Scheduler: Enqueueing reaction Watchdog reaction 1, which has level 2.
    DEBUG: Scheduler: Accessing triggered reactions at the level 2 with index 1.
    DEBUG: Scheduler: Index for level 2 is at 1.
    DEBUG: Worker 1: Done with reaction Watchdog.w reaction 0.
    DEBUG: Worker 1 is out of ready reactions.
    DEBUG: Scheduler: Worker 1 is the last idle thread.
    DEBUG: Start of rxn queue at 1 is 0x0
    DEBUG: Start of rxn queue at 2 is 0x1440089b8
    DEBUG: Scheduler: Notifying 2 workers.
    DEBUG: Scheduler: New number of idle workers: 0.
    DEBUG: Scheduler: Worker 1 popping reaction with level 2, index for level: 1.
    DEBUG: Worker 1: Got from scheduler reaction Watchdog reaction 1: level: 2, is input reaction: 0, and deadline -9223372036854775808.
    LOG: Worker 1: Invoking reaction Watchdog reaction 1 at elapsed tag (11000000000, 0).
    Watcher reactor produced an output. 1
    >>>>>> Taking a long time to process that output!
    DEBUG: Scheduler: Worker 0 acquired the scheduling semaphore.
    DEBUG: Scheduler: Worker 0 popping reaction with level 2, index for level: 0.
    DEBUG: Worker 0: Got from scheduler reaction Watchdog.w reaction 2: level: 2, is input reaction: 0, and deadline -9223372036854775808.
    LOG: Worker 0: Invoking reaction Watchdog.w reaction 2 at elapsed tag (11000000000, 0).
    FATAL ERROR: Watchdog expired 1 times. Expected at least 2.

As this feature hasn't been properly documented yet, we'll keep it that way until this has been addressed. For now, the tests will be marked as failing.