Add new coverage for Unit Testing (coming to dbt-core in 1.8)

[graciegoheen]

• we should update our vocabulary so that:
  • "tests" means "both data tests and unit tests"
  • any time we want to refer to JUST data tests, we're explicitly calling them "data tests"
• any new rules (or changes to current ones) to account for unit tests:
  • should unit tests count for test coverage on a model
  • any way to flag models that should be unit tested (that don't have a unit test)