Add references to test cases

[nikhilwoodruff]

Hi there!

I really enjoy OpenFisca, but I recently encountered an issue.

Here is what I did:

Add a reference attribute to a test case, to explain where the expected values came from.

Here is what I expected to happen:

The test to be unaffected.

Here is what actually happened:

A ValueError was raised when executing the test script: ValueError: Unexpected keys {'reference'} in test

Context

I identify more as a:

• Developer (I create tools that use the existing OpenFisca code).

Thanks - happy to file a PR if this is desirable.

[benjello]

You can use the description to add some info about the test case ...

[MaxGhenis]

One example of this is https://github.com/PolicyEngine/openfisca-us/pull/387, where we're creating test cases for a benefit program from this third party calculator.