Schema test macro referencing package macro errors in parse-time rendering

[jtcohen6]

Describe the bug

Related to #3272 + #3280. The schema test macro is now in the rendering context for schema tests, which is great! Now, however, if a schema test macro depends on a macro from a package, it raises an error.

Reproduce

packages.yml:

packages:
  - package: fishtown-analytics/dbt_utils
    version: 0.6.4

macros/test/my_test.sql:

{% macro test_my_test(model) %}
    select {{ dbt_utils.current_timestamp() }}
{% endmacro %}

models/my_model.sql:

select 1 as fun

models:
  - name: my_model
    tests:
      - my_test

In develop and v0.19.2-rc1, I'm seeing the following error:

Compilation Error in test my_test_my_model_ (models/schema.yml)
  'dbt_utils' is undefined

  > in macro test_my_test (macros/my_test.sql)
  > called by test my_test_my_model_ (models/schema.yml)

This stacktrace should feel very familiar:

File "/usr/local/Cellar/dbt@0.19.2-rc1/0.19.2rc1_1/libexec/lib/python3.8/site-packages/dbt/parser/schemas.py", line 555, in parse_node
    node = self._parse_generic_test(
  File "/usr/local/Cellar/dbt@0.19.2-rc1/0.19.2rc1_1/libexec/lib/python3.8/site-packages/dbt/parser/schemas.py", line 467, in _parse_generic_test
    self.render_test_update(node, config, builder)
  File "/usr/local/Cellar/dbt@0.19.2-rc1/0.19.2rc1_1/libexec/lib/python3.8/site-packages/dbt/parser/schemas.py", line 505, in render_test_update
    get_rendered(

We'll need to catch this UndefinedError, extend the parse-time schema test rendering context, or something else I haven't thought of.

[jtcohen6]

Possibly related: we're still seeing error #3229 for tests defined in packages. (Thanks @MartinNowak for the report, I just confirmed locally.)

Defined in a package:

{% macro test_valid_code(model) %}
    select count(*) from (
        select * from {{ model }}
        left join {{ ref('model_b') }}
    )
{% endmacro %}

Defined in the root project: model_a, model_b, and

version: 2

models:
  - name: model_a
    tests:
      - core_pkg.valid_code

Error:

Compilation Error in test core_pkg_valid_code_model_a_ (models/schema.yml)
  dbt was unable to infer all dependencies for the model "core_pkg_valid_code_model_a_".
  This typically happens when ref() is placed within a conditional block.

  To fix this, add the following hint to the top of the model "core_pkg_valid_code_model_a_":

  -- depends_on: {{ ref('model_b') }}

  > in macro test_valid_code (macros/tests.sql)
  > called by test core_pkg_valid_code_model_a_ (models/schema.yml)
  > called by test core_pkg_valid_code_model_a_ (models/schema.yml)