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.
Tree-SHA512: 72e04bc21462ebd0cb346fd1fe0540da454acfbad41923a0b06ea2317e9045b68e58f9adb02d8200891aca89a9d03a022eb72282aeb31a3b3afe7c6843a4b450
Backport of Core PR12904
https://github.com/bitcoin/bitcoin/pull/12904/
Depends on D3234
Completes T446