Add dbt column tests

Previously, when you built out the staging models, your work was focused on properly documenting column descriptions for each staging model. The same yaml file used to document column descriptions has another use: to add dbt data tests.

Data tests are used to guardrail against data drift on granularity and data quality, acting as an early warning system. The four out-of-the-box data tests are: unique, not_null, accepted_values, and relationships.

In this exercise, you will get hands-on practice for applying three out of these four tests.