Test parsing context: package + macro deps + dispatch, oh my

[jtcohen6]

Follow-on from #3345 (I know, I hoped we were done with this one too!)

For better and for worse (definitely better!), dbt-utils does a lot of things with macro nesting and dispatching within custom schema test definitions. So, it's pretty good surface area for testing this functionality. This is on me for not testing #3345 against dbt-utils before approving #3345.

The problem seems to be when a package macro's dispatched implementation calls, in turn, another dispatched package macro. The second macro isn't included in the test parsing context. Currently, we're seeing this issue with dbt_utils.test_recency, the default implementation of which in turn calls the dbt_utils.current_timestamp macro:

Encountered an error:
Compilation Error in test dbt_utils_recency_test_recency_day__today__1 (models/schema_tests/schema.yml)
  'dict object' has no attribute 'current_timestamp'
  > in macro test_recency (macros/schema_tests/recency.sql)
  > called by macro default__test_recency (macros/schema_tests/recency.sql)
  > called by test dbt_utils_recency_test_recency_day__today__1 (models/schema_tests/schema.yml)

I've opened a draft PR (#3363) that adds a reproduction case to 008_schema_tests_test. I'm hopeful that this is as simple as altering the logic we've already put in place to recurse through depends_on.macros and add to the test parsing context.

Once we have a fix for that, we'll also need to cherry-pick it (and the fix from #3345) onto 0.19.latest for inclusion in v0.19.2-rc2.

[jtcohen6]

After discussing with @gshank, we've realized:

• adapter.dispatch() needs to be statically analyzable in order to establish macro dependencies (in depends_on.macros)
• dispatch() is written in python (good!), but it's possible to pass it non-static arguments (not as good!) This is a common pattern established by dbt-utils. The value of packages is statically knowable at parse time,

{% macro _get_utils_namespaces() %}
  {% set override_namespaces = var('dbt_utils_dispatch_list', []) %}
  {% do return(override_namespaces + ['dbt_utils']) %}
{% endmacro %}

{%- macro type_string() -%} 
  {{ return(adapter.dispatch('type_string', packages = dbt_utils._get_utils_namespaces())()) }}
{%- endmacro -%}

For now, we're thinking about ways to call/render the _get_utils_namespaces() macro from python at parse time. In the next minor version, we should formalize the _get_utils_namespaces() convention (which other package maintainers have adopted) by moving this into a python method that expects a variable name following a set convention (<package_name>_dispatch_list). This would require no change for end users, and minimal change for package maintainers.