[Wilt Chamberlain] Render severity config for tests

[drewbanin]

Feature

Feature description

As of 0.14.0, test severity can be configured with a syntax like:

version: 2

models:
  - name: my_models
    columns:
       - name: id
          tests:
            - unique:
                 severity: error

If possible, we should consider rendering the severity field with a macro context. This would make it possible to implement complex test logic in an environment-aware way. Eg:

version: 2

models:
  - name: my_models
    columns:
       - name: id
          tests:
            - unique:
                 severity: "{{ 'error' if var('strict', false) else 'warn' }}"

Conceivably, users could put logic like this into macros, then use those macros in their severity specifications.

We've spoken about what ends up in the rendering context in #1255, so if this is mired in some larger conversation about the conversation context, then we can tackle this in a future release. If it's straightforward to implement now, I think it would be a great addition to the Wilt Chamberlain release of dbt.

[drewbanin]

Desired context (global):

• target
• CLI vars
• modules
• run started at / invocation_id