[CT-3323] Allow singular tests to be documented

[gshank]

Description

Currently, you can document pretty much everything but a data test - if you try, it fails on a parsing error. This ticket is for singular tests. A separate ticket (#2578) addresses generic tests.

Currently we don't have a "tests" section in schema files. In order to allow setting descriptions in yaml files we should add one. (As a side issue, the SingularTestNodes use the TestConfig, which is not particularly valid for SingularTestNodes. Maybe we need a separate SingularTestConfig.)

https://docs.getdbt.com/docs/build/tests#singular-tests

Acceptance Criteria

• You can add a description to a singular test in a yaml file
• You can use a docs block to capture your test description
• That description shows up in the docs site

Considerations

• Do we need to exclude this from state modified?
• New section in schema files and new schema parser
• Don't have a test schema parser
• Need yml to be able to have a description

[MichelleArk]

Since singular tests aren't explicitly linked to models in schema.yml files already, we would need to introduce a new top-level 'tests' resource in order to parse descriptions:

# top-level
tests:
  - name: my_super_cool_singular_test
    description: The order_id is unique for every row in the orders model
    config:
      error_if: ">10"
  - name: my_super_cool_singular_test2
    description: '{{ doc("not_null_test") }}'

cc @graciegoheen @jtcohen6

[graciegoheen]

Yep - this feels similar to how we handle descriptions for seeds or macros! Only adjustment I would say is data_tests: instead of tests: (since we're introducing unit tests in 1.8):

# top-level
data_tests:
  - name: my_super_cool_singular_test
    description: The order_id is unique for every row in the orders model
    config:
      error_if: ">10"
  - name: my_super_cool_singular_test2
    description: '{{ doc("not_null_test") }}'