Fix leaky tests detected by nextest

[TheQuantumPhysicist]

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When running unit tests with nextest, it detects leaky tests, which are defined as tests that don't clean-up properly when they finish. We should investigate this.

Note: For some reason, these are only detected on Windows. For me, I saw them when running the tests in git bash. This isn't found/detected by Linux or Mac.

[discosultan]

Also experienced similar warnings on Windows with Git Bash. The warnings are intermittent and not trivially reproducible. Here are a few examples:

LEAK [   0.138s] common chain::transaction::signature::tests::mutate_none::case_1
LEAK [   0.187s] crypto hash::tests::test_hash_sha3_512
LEAK [   0.184s] crypto hash::tests::test_ripemd160
LEAK [   0.184s] crypto hash::tests::test_ripemd160_stream