[Misc] [CI/Build] Speed up block manager CPU-only unit tests ~10x by opting-out of GPU cleanup

[cadedaniel]

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#3631 adds a global cleanup which empties the torch cache among other things. For CPU-only unit tests, this is unnecessary and actually causes a big slowdown for CPU-only tests due to loading torch. My dev machine has a slow EBS connection so loading torch is like ~10x time of a small CPU test.

This PR adds a pytest fixture to disable global cleanup. In your conftest (in some subdirectory) you can opt-out of global cleanup by overriding it as follows:

@pytest.fixture()
def should_do_global_cleanup_after_test() -> bool:
    return False