Fix fake event test flake

[adeebshihadeh]

Summary

• isolate test_synced_pub_sub with unique endpoints and deterministic publisher/subscriber ordering
• join the helper process after killing it
• require FIFO event waits to be readable before treating poll wakeups as signals

Test plan

• ./test.sh
• 300x .venv/bin/python -m unittest msgq.tests.test_fake