test: fix timeout decrease in feature_assumevalid

Author: brunoerg

test/functional/feature_assumevalid.py

@@ -90,7 +90,7 @@ 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