Separate tests' `json` declaration as Exercise
Separate tests' `json` declaration as Exercise

Since the tests cover a testing tool, separating the Exercise [phase] from
the Verification phase is tricky.

This commit pulls out in-line `json` declarations into the Exercise
phase, and separates them from the Verification phase with lines of
whitespace.

[phase]: https://robots.thoughtbot.com/four-phase-test

thoughtbot/json_matchers,seanpdoyle/json-matchers