e36a0c0 [qa] Ensure bitcoind processes are cleaned up when tests end (Suhas Daftuar)
Pull request description:
When tests fail (such as due to a bug in the test, race condition, etc), it's possible that we could follow code paths that bypass our normal node shutdown that occurs in `TestNode.stop_node`. Add a destructor to `TestNode` that cleans this up.
Backport of Core PR12904
Depends on D3234