test: remove assert_blockchain_height

Summary:
> This simplifies the code and solves intermittent timeouts caused by commit rABC0c0123b0aa87

Note: I have not yet seen an intermittent timeout happen, but the conditions for this to happen exist after D10896

This is a backport of [[bitcoin/bitcoin#21117 | core#21117]]

Test Plan: `ninja check-functional`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10950

test/functional/feature_assumevalid.py

@@ -29,7 +29,6 @@
       block 200. node2 will reject block 102 since it's assumed valid, but it
       isn't buried by at least two weeks' work.
 """
-import time
 
 from test_framework.blocktools import create_block, create_coinbase
 from test_framework.key import ECKey
@@ -81,25 +80,6 @@ def send_blocks_until_disconnected(self, p2p_conn):
                 assert not p2p_conn.is_connected
                 break
 
-    def assert_blockchain_height(self, node, height):
-        """Wait until the blockchain is no longer advancing and verify it's reached the expected height."""
-        last_height = node.getblock(node.getbestblockhash())['height']
-        timeout = 10
-        while True:
-            time.sleep(0.25)
-            current_height = node.getblock(node.getbestblockhash())['height']
-            if current_height != last_height:
-                last_height = current_height
-                if timeout < 0:
-                    assert False, "blockchain too short after timeout: {}".format(
-                        current_height)
-                timeout -= 0.25
-                continue
-            elif current_height > height:
-                assert False, "blockchain too long: {}".format(current_height)
-            elif current_height == height:
-                break
-
     def run_test(self):
         p2p0 = self.nodes[0].add_p2p_connection(BaseNode())
 
@@ -189,7 +169,8 @@ def run_test(self):
 
         # Send blocks to node0. Block 102 will be rejected.
         self.send_blocks_until_disconnected(p2p0)
-        self.assert_blockchain_height(self.nodes[0], 101)
+        self.wait_until(lambda: self.nodes[0].getblockcount() >= 101)
+        assert_equal(self.nodes[0].getblockcount(), 101)
 
         # Send all blocks to node1. All blocks will be accepted.
         for i in range(2202):
@@ -202,7 +183,8 @@ def run_test(self):
 
         # Send blocks to node2. Block 102 will be rejected.
         self.send_blocks_until_disconnected(p2p2)
-        self.assert_blockchain_height(self.nodes[2], 101)
+        self.wait_until(lambda: self.nodes[2].getblockcount() >= 101)
+        assert_equal(self.nodes[2].getblockcount(), 101)
 
 
 if __name__ == '__main__':