Smarter Unit Test Execution

[cmgoffena13]

Right now all unit tests run during a plan unless the --skip-tests flag is used.

We already have the plan which shows us which models are affected by the changes. It makes sense to trim the unit tests that run during a plan to only the models that are affected by applying the plan.

I am not sure what behavior should happen for the --skip-backfill / --dry-run flags, but my initial thought is to skip all tests for this flag as well since it's considered a no-op operation.

To have backwards compatibility, we can add in an --all-tests flag to the plan command. I can see this being useful for CI dry-runs as well as edge cases that may show up.

[cmgoffena13]

After thinking about it: the --skip-backfill / --dry-run flags already run all tests, so to make behavior more predictable, it makes more sense to just limit tests to model changes in a dry-run too. Keeping the --all-tests flag for backwards compatibility though.