Mocha lacks built-in support [1] for writing parameterized tests and the
suggested solution [2] involves a bunch of boilerplate* which has IMO
resulted in different styles of parameterized tests in our codebase and
not having parameterized tests when they would be useful to attain more
complete coverage.

This adds a helper inspired by [3] for writing parameterized tests and
switches several existing places in our code to use it.

* Though less with ES2015 syntax.

[1] https://github.com/mochajs/mocha/issues/1454
[2] https://mochajs.org/#dynamically-generating-tests
[3] https://github.com/lawrencec/Unroll