Support selecting a single test or file on assert

[mariana-s-fernandes]

I would like the assert command to support selecting a single file or test so we could make our testing more modular.
Looker sdk supports this: https://developers.looker.com/api/explorer/4.0/methods/Project/run_lookml_test

[joshtemple]

Hey @mariana-s-fernandes! I think this is a great idea! This isn't something we're likely to prioritize soon, but I think it would be a pretty straightforward change to make if you or someone else would be willing to submit a PR.

The necessary code is here, you can see we get all tests and then pass them into the validate method.

Probably we would prefer to pass in some kind of a filter from the CLI, or even specific test names.