Hermetic RK script testing

[htuch]

Currently it's plausible to test RK scripts via Starlark unit testing (https://docs.bazel.build/versions/master/skylark/testing.html#for-testing-starlark-utilities), but this doesn't capture the behavior of the RK API surface.

Opening an issue to track adding support for Starlark unit testing combined with RK in some digestible framework.

[phlax]

this would be great!

[phlax]

the github api/events also come into play here

it might also be helpful if there were a repo with "auto-bless" (perhaps for members) to test/dev against github events