test.py calls `uninstall()` and `stop()` concurrently from exit
artifacts, and `uninstall()` internally calls `stop()`. This leads to
premature releasing of IP addresses from `uninstall()` (returning IPs to
the pool) while the servers using those IPs are still stopping. Then a
server might obtain that IP from the pool and fail to start due to
"Address already in use".
Put a lock around the body of `stop()` to prevent that.
Fixes: scylladb/scylladb#15755Closesscylladb/scylladb#15763
059d647ee5