ral-check: add a test harness for unit testing providers

[lutter]

There should be a separate tool, ral-check which can be used to unit test providers. The testing will be black box testing, i.e. solely interact with the provider through its API. Writing tests should be very simple, ideally data driven.

A sketch of what such data driven tests look like is in this gist

Running tests should then just be a matter of executing ral-check test.yaml