Write small test suite that ensures key variables produce impacts

[anth-volk]

This is meant to complement PolicyEngine/policyengine-core#252. The root of the issue behind that PR was an incorrectly written line that, when executed, did not apply a reform correctly, causing a reform to generate the same impact as the baseline. We could use 5-10 tests for 5-10 super common variables of different types, and would just test to make sure that there is, in fact, an impact from each of these. Alternatively, we create dummy variables with formulas that give very predictable outputs and test for those exact outputs.

Testing for this in core would not be as effective as within the country package itself, as it could catch a wide net of unintended bugs that might impact the country model.

Duplicate of PolicyEngine/policyengine-us#4925.