Configurable description in yaml config for generic tests

Describe the feature

Add the ability to define descriptions in yaml files. Support the ability to use the rendered values of the test args ({{ model }} and {{ column_name }} by default)

Consider the following example:

# macros/reusable_tests.sql
{% test not_null(model, column_name), adapter = 'default' %}

    select * from {{ model }}
    where {{ column_name }} is null

{% endtest %}

# macros/schema.yml
tests:
 - name: not_null
   adapter: default
   description: "Assert that {{ column_name }} is not null in {{ model }}"
   args:
     - name: model
       description: "The model to test"
     - name: column_name
       description: "The column or expression to test"

Additional context in #1173.

[joellabes]

I still want this! Mostly for the aliasing reasons discussed in #3815

[github-actions[bot]]

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