test(tiers): XdistSubprocessTests checks for a leftover probe before taking the lock #1758

Describe the bug
tests/test_tier_guard_xdist.py: XdistSubprocessTests.setUp checks probe_path.exists() at lines 176-179 before it takes the exclusive _tier_guard_lock at line 189.

Under --dist load, the class's two tests can land on different workers. One of them can then see the other's live probe and fail with "already exists — a previous run did not clean up", even though nothing went wrong.

#1757 adds a third writer of the same probe file, ProbeWorkerCrashTests. That class checks only after taking the lock.

Expected behavior
The existence check runs under the lock, so a concurrent probe is waited for, not reported as a leftover.

Additional context

• Found by the worker and the Sonnet reviewer on test(tiers): keep the xdist probe out of recursive collection (#1756) #1757.
• Not seen to fail: 8 xdist runs and six -n 3 repeats were all clean.