test: Plug memory leaks and stack-use-after-scope

Summary:
Backport of core PR12477
https://github.com/bitcoin/bitcoin/pull/12477/files

The modifications in checkqueue_tests.cpp are already part of our
codebase.

The modifications of test_bitcoin are only required to clear memory
from avalanche_tests and DoS_tests. Avalanche tests use
std::_unique_ptr, making this fix useless, and so does DoS tests
since D2919.

Depends on D2919

Test Plan:

./src/bench/bitcoin-bench

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2918