[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/fundrawtransaction.py

@@ -451,8 +451,7 @@ class RawTransactionsTest(BitcoinTestFramework):
         self.stop_node(0)
         self.stop_node(2)
         self.stop_node(3)
-        self.nodes[1].encryptwallet("test")
-        self.bitcoind_processes[1].wait(timeout=BITCOIND_PROC_WAIT_TIMEOUT)
+        self.nodes[1].node_encrypt_wallet("test")
 
         self.nodes = self.start_nodes(self.num_nodes, self.options.tmpdir)
         # This test is not meant to test fee estimation and we'd like