Merge bitcoin#21084: test: fix timeout decrease in feature_assumevalid

0d39b58 test: fix timeout decrease in feature_assumevalid (Bruno Garcia)

Pull request description:

  This PR fixes the timeout decrease in assert_blockchain_height function.

ACKs for top commit:
  practicalswift:
    cr ACK 0d39b58: patch looks correct
  theStack:
    ACK 0d39b58 ⏲️

Tree-SHA512: ae3c83420b4de4ad41f1b20b6e77c3a26a8c5ac4fb136b2645fde119545a413c61312f76a16473141774bc955d30ac4fc86e5ca6cf729f8978a17d0dab520feb

test/functional/feature_assumevalid.py

@@ -88,7 +88,7 @@ def assert_blockchain_height(self, node, height):
                 last_height = current_height
                 if timeout < 0:
                     assert False, "blockchain too short after timeout: %d" % current_height
-                timeout - 0.25
+                timeout -= 0.25
                 continue
             elif current_height > height:
                 assert False, "blockchain too long: %d" % current_height