Properly fix Git error in tests.

[reitermarkus]

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[reitermarkus]

@MikeMcQuaid, I think I found the right fix. We also need to clear the cache after each Git spec, not just before each one.

[MikeMcQuaid]

@reitermarkus Thanks, sounds good. Can you also remove the HOMEBREW_TEST_GENERIC_OS addition?

[MikeMcQuaid]

(or raise an exception in that case instead of unsetting)

[MikeMcQuaid]

Thanks for the change 👏🏻