Merge #12904: [qa] Ensure bitcoind processes are cleaned up when tests end

Summary:
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

Test Plan:

make check
test_runner.py

Reviewers: jasonbcox, deadalnix, Fabien, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3235