test_varying_your_earnings should be refactored

[anth-volk]

The original aim of this test was to prevent a situation where a change to a country package, -core, or -api caused a certain type of bug that occurred whereby either the baseline or the reform situation contained NaN values, thereby causing NaN values when comparing the two, and then returning NaN values that crashed the front-end app. However, I think this test is too heavy, does too many things at once, and should be refactored into a proper integration test. It should also mock the database (something very few of the tests do).