unit tests: Avoid side effects #12428

To the best of my knowledge, none of the unit test cases has another test case as dependency. In fact, I consider it the safest and cleanest, if none of the unit tests relies on side effects from previous unit tests.

Bitcoin Core heavily relies on globals, which are exposed to all side effects of test cases (even in completely different test suites). They might cause non-deterministic bugs, such as #12424.

I suggest to run each test case in a fresh environment with cleanly initialized globals.

Running each test separately could also be used to run all of them in parallel (similar to how the functional tests are run in parallel)

Will push a branch soon