Allow data tests to be documented

Describe the feature

Currently, you can document pretty much everything but a data test. You also therefore cannot hide them from the docs site.

Open questions:

• Should they be under a tests: key, or a data_tests: key?

Not much else to say here!

[NicolasPA]

I think the ticket activity doesn’t reflect the demand for this feature, there has been at least 6 users asking about in on Slack this past year: https://getdbt.slack.com/archives/CBSQTAPLG/p1631304882432300

It might be because the wording used in the ticket has changed: schema test → generic test data test → singular test

I think custom tests are code as much as models and macros are, so documenting them is equally important.

@jtcohen6: I think a new tests: key in .yml files is exactly how we’d want to do this. Singular tests wouldn’t be too hard. Generic tests would be trickier, since the good version (IMO) allows for templated descriptions that includes the values of {{ model }}, {{ column_name }}, and potentially other arguments.

IMO, having the static description as is in written in the YAML without replacing the values would already be nice and useful, the more complex version could be kept for later.

[RobMcZag]

I would say that for me a tests: label would imply all kind of tests, including schema tests, but looking at the current naming it looks like the most reasonable. Current names related to tests:

• data tests are in the tests folder, not data-tests
• in dbt_projectwe specify a folder for test-paths, not data-test-paths