Merge bitcoin/bitcoin#26651: test: Avoid intermittent timeout in feature_assumevalid.py

fa34e5f test: Avoid intermittent timeout in feature_assumevalid.py (MarcoFalke)

Pull request description:

  Currently the test will spin up p2p connections in the beginning, then announce the headers to all nodes, but only send the blocks sequentially. This takes a long time, so when getting to the last node, it will have already timed out, while node1 is busy eating blocks. Example:

  ```
   node2 2022-12-06T19:31:35.419291Z [msghand] [net_processing.cpp:5783] [SendMessages] [net] Requesting block 2cfdb317b3b901f79e2d4f96339d0c0dffd8ef2685d324f62ab0e2fa3402430e (1) peer=0
   node2 2022-12-06T19:31:35.424784Z [msghand] [net.cpp:2776] [PushMessage] [net] sending getdata (577 bytes) peer=0
  [...]
   node2 2022-12-06T19:41:35.423257Z [msghand] [net_processing.cpp:5729] [SendMessages] Timeout downloading block 2cfdb317b3b901f79e2d4f96339d0c0dffd8ef2685d324f62ab0e2fa3402430e from peer=0, disconnecting
   node1 2022-12-06T19:41:35.438706Z [msghand] [net_processing.cpp:5783] [SendMessages] [net] Requesting block 6575919043306ed309014d0bd79814b4fab8afaa281e026d8cc3a1c4c2336fbc (1748) peer=0
   node2 2022-12-06T19:41:35.521253Z [net] [net.cpp:573] [CloseSocketDisconnect] [net] disconnecting peer=0
   node2 2022-12-06T19:41:35.630417Z [net] [net_processing.cpp:1532] [FinalizeNode] [net] Cleared nodestate for peer=0
  ```

  Fix this by only spinning up the p2p connection right before they are needed.

ACKs for top commit:
  jamesob:
    ACK fa34e5f ([`jamesob/ackr/26651.1.MarcoFalke.test_avoid_intermittent`](https://github.com/jamesob/bitcoin/tree/ackr/26651.1.MarcoFalke.test_avoid_intermittent))

Tree-SHA512: 7a1b114c07dfa30237c8cd8637dd6646c5c2dc2530c9de61db231738fddc800b620c31dc664237e33d35e951cf161f015fda593162efc9d85c5f68c6e37217d4

Author: MarcoFalke

test/functional/feature_assumevalid.py

@@ -83,8 +83,6 @@ def send_blocks_until_disconnected(self, p2p_conn):
                 break
 
     def run_test(self):
-        p2p0 = self.nodes[0].add_p2p_connection(BaseNode())
-
         # Build the blockchain
         self.tip = int(self.nodes[0].getbestblockhash(), 16)
         self.block_time = self.nodes[0].getblock(self.nodes[0].getbestblockhash())['time'] + 1
@@ -139,35 +137,33 @@ def run_test(self):
             self.block_time += 1
             height += 1
 
-        self.nodes[0].disconnect_p2ps()
-
         # Start node1 and node2 with assumevalid so they accept a block with a bad signature.
         self.start_node(1, extra_args=["-assumevalid=" + hex(block102.sha256)])
         self.start_node(2, extra_args=["-assumevalid=" + hex(block102.sha256)])
 
         p2p0 = self.nodes[0].add_p2p_connection(BaseNode())
-        p2p1 = self.nodes[1].add_p2p_connection(BaseNode())
-        p2p2 = self.nodes[2].add_p2p_connection(BaseNode())
-
-        # send header lists to all three nodes
         p2p0.send_header_for_blocks(self.blocks[0:2000])
         p2p0.send_header_for_blocks(self.blocks[2000:])
-        p2p1.send_header_for_blocks(self.blocks[0:2000])
-        p2p1.send_header_for_blocks(self.blocks[2000:])
-        p2p2.send_header_for_blocks(self.blocks[0:200])
 
         # Send blocks to node0. Block 102 will be rejected.
         self.send_blocks_until_disconnected(p2p0)
         self.wait_until(lambda: self.nodes[0].getblockcount() >= COINBASE_MATURITY + 1)
         assert_equal(self.nodes[0].getblockcount(), COINBASE_MATURITY + 1)
 
+        p2p1 = self.nodes[1].add_p2p_connection(BaseNode())
+        p2p1.send_header_for_blocks(self.blocks[0:2000])
+        p2p1.send_header_for_blocks(self.blocks[2000:])
+
         # Send all blocks to node1. All blocks will be accepted.
         for i in range(2202):
             p2p1.send_message(msg_block(self.blocks[i]))
         # Syncing 2200 blocks can take a while on slow systems. Give it plenty of time to sync.
         p2p1.sync_with_ping(960)
         assert_equal(self.nodes[1].getblock(self.nodes[1].getbestblockhash())['height'], 2202)
 
+        p2p2 = self.nodes[2].add_p2p_connection(BaseNode())
+        p2p2.send_header_for_blocks(self.blocks[0:200])
+
         # Send blocks to node2. Block 102 will be rejected.
         self.send_blocks_until_disconnected(p2p2)
         self.wait_until(lambda: self.nodes[2].getblockcount() >= COINBASE_MATURITY + 1)