Fix flaky Formulary test.

[reitermarkus]

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The CaskLoader and Formulary tests use the same tap migrations, but in opposite directions. I assume there is a small possibility that both migrations exist at the same time, resulting in an infinite loop between the two taps.

I added two tests to reproduce the flaky test. Will have a look later at how to best fix this. Simplest solution for now is to not use the same formula/cask name, but that doesn't really scale.