Merge bitcoin/bitcoin#27941: test: Fix intermittent issue in mining_getblocktemplate_longpoll.py

fa748c6 test: Fix intermittent issue in mining_getblocktemplate_longpoll.py (MarcoFalke)

Pull request description:

  Fixes bitcoin/bitcoin#26962

  Wait for the thread to have started and the RPC to have reached the node before continuing. Otherwise the test may run into a race.

  For example:

  ```
   test  2023-06-23T13:10:29.245000Z TestFramework (INFO): Test that introducing a new transaction into the mempool will terminate the longpoll
   node0 2023-06-23T13:10:29.245712Z [http] [httpserver.cpp:254] [http_request_cb] [http] Received a POST request for / from 127.0.0.1:43568
   node0 2023-06-23T13:10:29.245915Z [httpworker.3] [rpc/request.cpp:181] [parse] [rpc] ThreadRPCServer method=getblocktemplate user=__cookie__
   node0 2023-06-23T13:10:29.252594Z [http] [httpserver.cpp:254] [http_request_cb] [http] Received a POST request for / from 127.0.0.1:43568
   node0 2023-06-23T13:10:29.254545Z [httpworker.2] [rpc/request.cpp:181] [parse] [rpc] ThreadRPCServer method=getblockchaininfo user=__cookie__
   node0 2023-06-23T13:10:29.256530Z [http] [httpserver.cpp:254] [http_request_cb] [http] Received a POST request for / from 127.0.0.1:43568
   node0 2023-06-23T13:10:29.256741Z [httpworker.1] [rpc/request.cpp:181] [parse] [rpc] ThreadRPCServer method=sendrawtransaction user=__cookie__
   node0 2023-06-23T13:10:29.258033Z [httpworker.1] [validationinterface.cpp:213] [TransactionAddedToMempool] [validation] Enqueuing TransactionAddedToMempool: txid=38335600f2465c0f8bb2b86d5830a34851d86fa879800c0e1434ddfc78c42898 wtxid=c033cd3efd301c369d66cf759769159609471bd4f9efb3ee30e7209e57b74778
   node0 2023-06-23T13:10:29.258263Z [httpworker.1] [txmempool.cpp:660] [check] [mempool] Checking mempool with 1 transactions and 1 inputs
   node0 2023-06-23T13:10:29.258542Z [scheduler] [validationinterface.cpp:213] [operator()] [validation] TransactionAddedToMempool: txid=38335600f2465c0f8bb2b86d5830a34851d86fa879800c0e1434ddfc78c42898 wtxid=c033cd3efd301c369d66cf759769159609471bd4f9efb3ee30e7209e57b74778
   node0 2023-06-23T13:10:29.259549Z [http] [httpserver.cpp:254] [http_request_cb] [http] Received a POST request for / from 127.0.0.1:43568
   node0 2023-06-23T13:10:29.259745Z [httpworker.0] [rpc/request.cpp:181] [parse] [rpc] ThreadRPCServer method=decoderawtransaction user=__cookie__
   node0 2023-06-23T13:10:29.261066Z [http] [httpserver.cpp:254] [http_request_cb] [http] Received a POST request for / from 127.0.0.1:52690
   node0 2023-06-23T13:10:29.261803Z [http] [httpserver.cpp:254] [http_request_cb] [http] Received a POST request for / from 127.0.0.1:43568
   node0 2023-06-23T13:10:29.262770Z [httpworker.2] [rpc/request.cpp:181] [parse] [rpc] ThreadRPCServer method=getblocktemplate user=__cookie__
  ```

  (`sendrawtransaction` is called before `getblocktemplate`)

ACKs for top commit:
  jamesob:
    Github ACK bitcoin/bitcoin@fa748c6
  theStack:
    ACK fa748c6

Tree-SHA512: c67d9ec7c56e8a22c1a26a3c3d4d4a4bcc17e4282cad0d66561ba2abd6e92240cb028369b4edc6077ea34e8736c0294f6066381979aee22a6166580cea43729a

Author: fanquake

test/functional/mining_getblocktemplate_longpoll.py

@@ -41,7 +41,8 @@ def run_test(self):
 
         self.log.info("Test that longpoll waits if we do nothing")
         thr = LongpollThread(self.nodes[0])
-        thr.start()
+        with self.nodes[0].assert_debug_log(["ThreadRPCServer method=getblocktemplate"], timeout=3):
+            thr.start()
         # check that thread still lives
         thr.join(5)  # wait 5 seconds or until thread exits
         assert thr.is_alive()
@@ -55,14 +56,16 @@ def run_test(self):
 
         self.log.info("Test that longpoll will terminate if we generate a block ourselves")
         thr = LongpollThread(self.nodes[0])
-        thr.start()
+        with self.nodes[0].assert_debug_log(["ThreadRPCServer method=getblocktemplate"], timeout=3):
+            thr.start()
         self.generate(self.nodes[0], 1)  # generate a block on own node
         thr.join(5)  # wait 5 seconds or until thread exits
         assert not thr.is_alive()
 
         self.log.info("Test that introducing a new transaction into the mempool will terminate the longpoll")
         thr = LongpollThread(self.nodes[0])
-        thr.start()
+        with self.nodes[0].assert_debug_log(["ThreadRPCServer method=getblocktemplate"], timeout=3):
+            thr.start()
         # generate a transaction and submit it
         self.miniwallet.send_self_transfer(from_node=random.choice(self.nodes))
         # after one minute, every 10 seconds the mempool is probed, so in 80 seconds it should have returned