Merge #11077: [tests] fix timeout issues from TestNode

2b4ea52 [tests] fix timeout issues from TestNode (John Newbery)

Pull request description:

  Fixes a couple of bugs from the introduction of TestNode:

  - test scripts were no longer able to specify a custom timeout for
  starting a node. Therefore tests with nodes that take a long time to
  start up (eg pruning.py) would fail.
  - the test for whether a node has failed on start up was broken
  by changing 'assert x is None' to 'assert not x'. Since
  subprocess.poll() can return None (indicating the node is still running)
  or 0 (indicating the node exited with return code 0), this was a
  regression.

Tree-SHA512: 42a62a5459eea2e5d83b44dae2a5ccc7b15eb7fef8f8745ff04884dbba8f79d66ffdd65c67d37f6865b36da3f522bcdd0d6ea99861d7ce86dd8a56dc29cd643f