[tests] Introduce TestNode

TestNode is a class responsible for all state related to a bitcoind node
under test. It stores local state, is responsible for tracking the
bitcoind process and delegates unrecognised messages to the RPC
connection.

This commit changes start_nodes and stop_nodes to start and stop the
bitcoind nodes in parallel, making test setup and teardown much faster.

Github-Pull: #10711
Rebased-From: 7897338918

test/functional/getblocktemplate_longpoll.py

@@ -17,7 +17,7 @@ class LongpollThread(threading.Thread):
         self.longpollid = templat['longpollid']
         # create a new connection to the node, we can't use the same
         # connection from two threads
-        self.node = get_rpc_proxy(node.url, 1, timeout=600)
+        self.node = get_rpc_proxy(node.url, 1, timeout=600, coveragedir=node.coverage_dir)
 
     def run(self):
         self.node.getblocktemplate({'longpollid':self.longpollid})