Custom data test blocks

[drewbanin]

Describe the feature

dbt should provide a first-class block for custom data tests. This would allow tests to be configured alongside models, for instance.

Example syntax:

{% test my_test_name %}

{{ config(...) }}

select ...

{% endtest %}

Beyond the addition of this tag, custom data tests should remain working how they have historically worked.

[clrcrl]

Note that this is the same syntax you've described for schema tests in #1173

[github-actions]

This issue has been marked as Stale because it has been open for 180 days with no activity. If you would like the issue to remain open, please remove the stale label or comment on the issue, or it will be closed in 7 days.