Merge bitcoin/bitcoin#23979: test: wait for rather than assert presence of file in startupnotify test

96eb009 test: wait rather than assert presence of file in startupnotify test (fanquake)

Pull request description:

  Should fix #23967.

ACKs for top commit:
  brunoerg:
    crACK 96eb009
  kristapsk:
    utACK 96eb009

Tree-SHA512: 9107970e45c027cfc6c6cbfcfd5a7d9f5956259bbbb11f5b180c3947126e42e62c0f8ffd69cf7b39b51c9c5b4fedbb753839d59aebe876be68c1484bb6065819

Author: MarcoFalke

test/functional/feature_startupnotify.py

@@ -26,7 +26,7 @@ def run_test(self):
 
         self.log.info("Test -startupnotify command is run when node starts")
         self.restart_node(0, extra_args=[f"-startupnotify=echo '{FILE_NAME}' >> {NODE_DIR}/{FILE_NAME}"])
-        assert os.path.exists(tmpdir_file)
+        self.wait_until(lambda: os.path.exists(tmpdir_file))
 
         self.log.info("Test -startupnotify is executed once")
         with open(tmpdir_file, "r", encoding="utf8") as f: