Fix source test arg rendering (#2114)

[beckjake]

resolves #2114

Description

With this PR, dbt now gives an error on tests in sources of the schema.yml section.

To get to the ultimately desired behavior, see #2149 - to be resolved in 0.17.0, probably.

This is a breaking change! We used to render args in the schema-yaml-rendering-context, which was incorrect and didn't match the behavior in models.

I'm left with the uncomfortable feeling that someone out there is using {{ invocation_id }} or {{ run_started_at }} in parameters to their tests (why else would we do this?).

We could alternatively generate a full runtime context and re-render wrapped SQL. I don't think that's the worst idea of all time, but it might be slower. Definitely a hack!

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[drewbanin]

The existing behavior is just... so.... wonky here. The fact that this works:

sources:
    - name: snowplow
      tables:
          - name: events
            columns:
                - name: number
                  tests:
                      - custom_test:
                          arg: var('abc', 'PROVIDED')

but that this doesn't:

                      - custom_test:
                          arg: "{{ var('abc', 'PROVIDED') }}"

is just terrible. So, this is a great change for consistency in compilation, but I am very much looking forward to working on #2149 in the future!

Nice work on this one, let's ship it