Refactor into distinct distribution and provider conformance tests

[negz]

Description of your changes

This allows parties interested in conformance to run a different Sonobuoy plugin depending on whether they want to test a Crossplane distribution or a Crossplane provider for conformance.

I have:

• [x] Read and followed conformance's contribution process.
• [x] Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

I've run the tests against a local kind cluster (though the provider tests are currently a no-op).