test: add missing sync to feature_fee_estimation.py

Fixes a race between node 1 catching up with the chain and mining a
new block in the sanity_check_rbf_estimates subtest.

Author: mzumsande

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):