[FR] Deprecate `CSVReporter`

**Is your feature request related to a problem? Please describe.**
A description of how to deprecate `CSVReporter` once and for all.

**Describe the solution you'd like**
* change "output_test_helper.cc" to parse JSON rather than CSV
* add a [tool to parse the JSON output into a CSV](https://gist.github.com/dominichamon/fd0570afb67907be49228f5fff3b33f8)
* remove `CSVReporter` and all references to it (just make it build and pass tests)

**Describe alternatives you've considered**
not deprecating it?

**Additional context**
this would cause a major version bump to v2. we have a v2 branch in which some of the above has been attempted but it is very stale.