[CT-2921] Create special unit testing Config object

[gshank]

The initial unit testing branch specifies TestConfig in model_config.py, but actually uses NodeConfig (from ParsedNode) in the UnitTestNode. Make a custom UnitTestConfig with appropriate fields and use in UnitTestNode.

Is this actually required? We're now selecting against standard model nodes, and the database/schema/identifier is also coming from the model nodes. So it's not clear what we would need from a specialized Config object.

[graciegoheen]

Do we want to support these configs?

• tags (test specific markers -> "flaky" "exclude")
• meta (on every other resource type)

[MichelleArk]

From refinement:

• this config (supporting tags & meta to start) should support jinja rendering for consistency
• this config should apply at the node-level (from schema.yml files) and also be configurable hierarchically from dbt_project.yml, with a new top-level unit block in dbt_project.yml