Merge #10711: [tests] Introduce TestNode

789733891 [tests] Introduce TestNode (John Newbery)

Pull request description:

  Continues #10082

  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.

  On my vm, this changeset reduces total test_runner runtime for the base set of tests
  (including building the cache) from 263s to 195s (a 25% speedup). Note that the time
  reported by test_runner does not include time spent building the cache: