Fix output of test_lowlevel tests in case of timeout (#6136)

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

Author: ogayot

tests/testserver/server.py

@@ -115,7 +115,8 @@ def _accept_connection(self):
 
     def __enter__(self):
         self.start()
-        self.ready_event.wait(self.WAIT_EVENT_TIMEOUT)
+        if not self.ready_event.wait(self.WAIT_EVENT_TIMEOUT):
+            raise RuntimeError("Timeout waiting for server to be ready.")
         return self.host, self.port
 
     def __exit__(self, exc_type, exc_value, traceback):