test/librbd/test_notify.py: force line-buffered output

"master" and "slave" invocations are intended to run in parallel and
coordinate between themselves.  Ensure that their respective output is
properly timestamped and ordered in teuthology.log file.

Fixes: https://tracker.ceph.com/issues/70831
Signed-off-by: Ilya Dryomov <[email protected]>

Author: idryomov

src/test/librbd/test_notify.py

@@ -1,4 +1,5 @@
 #!/usr/bin/python3
+import functools
 import os
 import sys
 import time
@@ -13,6 +14,8 @@
                  RBD_FEATURE_FAST_DIFF,
                  RBD_FLAG_OBJECT_MAP_INVALID)
 
+print = functools.partial(print, flush=True)
+
 POOL_NAME='rbd'
 PARENT_IMG_NAME='test_notify_parent'
 CLONE_IMG_NAME='test_notify_clone'