test: cluster: fix syslog listener datagram ordering race

UnixSockerListener used ThreadingUnixDatagramServer, which spawns a
new thread per datagram. The notification barrier in get_lines() relies
on all prior datagrams being handled before the notification. With
threading, the notification handler can win the lock before an audit
entry handler, so get_lines() returns before the entry is appended.
clear_audit_logs() then clears an incomplete buffer, and the late
entry leaks into the next test's before/after diff.

Switch to sequential UnixDatagramServer. The server thread now handles
datagrams in kernel FIFO order, so the notification is always processed
after all preceding audit entries.

Refs SCYLLADB-1277

test/cluster/test_audit.py

@@ -345,7 +345,7 @@ class UnixSockerListener:
                 elif data != "Initializing syslog audit backend.":
                     self.server.parent_instance.lines.append(data)
 
-    class UnixDatagramServer(socketserver.ThreadingUnixDatagramServer):
+    class UnixDatagramServer(socketserver.UnixDatagramServer):
         def __init__(self, socket_path, handler, parent_instance, lock):
             self.parent_instance = parent_instance
             self.mutex = lock