add description tag to test definition

### Describe the feature
Would like to add description to tests.

```
tests:
- dbt_utils.expression_is_true:
name: test_name
description: “test_description"
expression: '= X'
config:
where: “X= N"
```

### Describe alternatives you've considered
Using name as declarative as possible of whats being tested is not enough.

### Additional context
As the package can store test results in database, table description would be nice.

### Who will this benefit?
Better understanding for any test being executed. Normally 6 months after deployment no dev-team members remembers the why. Having a more declarative way of the intent on test will help maintainability in the long run

### Are you interested in contributing this feature?
Have never contributed, and not even know where to start, if some guidance is given then definitely.

Start by locating how dbt-utils test definitions are parsed and how test results are stored in the database; the issue names no files or tests. Confirm the expected behavior for the YAML description field and verify that the description is retained with the resulting test metadata.

Written by the indexing model from the issue text.