Merge bitcoin/bitcoin#31016: test: add missing sync to feature_fee_estimation.py

a1576ed test: add missing sync to feature_fee_estimation.py (Martin Zumsande)

Pull request description:

  This fixes a race:

  - In the `test_estimate_dat_is_flushed_periodically` subtest, node 0 is isolated and creates 10 blocks (no sync).
  - In `clear_estimates` the nodes are reconnected (but we don't wait for them to sync!)
  - In the `sanity_check_rbf_estimates` subtest, node 1 generates another block and syncs with the other nodes. The sync fails if the generated block is at the same height as the tip of node 0.

  Fix this by adding a sync to `clear_estimates`.

  Fixes #30990
  Fixes #30640

ACKs for top commit:
  maflcko:
    lgtm ACK a1576ed
  tdb3:
    code review ACK a1576ed

Tree-SHA512: 608ba619cacb4ff3a1ea934e03286f18c96afeebf06439334d40bff72025bd7bcc2c1093dae1824b30a37d3ac3ea569bc3118c33c0ca51610592aa1b4f420840

Author: fanquake

test/functional/feature_fee_estimation.py

@@ -398,6 +398,7 @@ def clear_estimates(self):
         self.start_node(0)
         self.connect_nodes(0, 1)
         self.connect_nodes(0, 2)
+        self.sync_blocks()
         assert_equal(self.nodes[0].estimatesmartfee(1)["errors"], ["Insufficient data or no feerate found"])
 
     def broadcast_and_mine(self, broadcaster, miner, feerate, count):