Merge bitcoin/bitcoin#21777: test: Fix feature_notifications.py intermittent issue

fa4aec2 test: Fix feature_notifications.py intermittent issue (MarcoFalke)

Pull request description:

  Fixes #21683

Top commit has no ACKs.

Tree-SHA512: 256806d82877477f4b3d795658f61127c0de4eff07216f6071f40a8ec1f5d43f3c587f35dd436d480dc261ef6646ac5547db104d22f3fcfeeb61bbdbe04bcc31

Author: MarcoFalke

test/functional/feature_notifications.py

@@ -166,6 +166,8 @@ def expect_wallet_notify(self, tx_details):
         # Should now verify contents of each file
         for tx_id, blockheight, blockhash in tx_details:
             fname = os.path.join(self.walletnotify_dir, notify_outputname(self.wallet, tx_id))
+            # Wait for the cached writes to hit storage
+            self.wait_until(lambda: os.path.getsize(fname) > 0, timeout=10)
             with open(fname, 'rt', encoding='utf-8') as f:
                 text = f.read()
                 # Universal newline ensures '\n' on 'nt'