Add dbt-unit-testing

[will-warner]

Description

Dbt Unit Testing is a dbt package that provides support for unit testing in dbt.

You can test models independently by mocking their dependencies (models, sources, snapshots, seeds).

Raising on behalf of the package authors as per issue.

Link to your package's repository: here.

Checklist

This checklist is a cut down version of the best practices that we have identified as the package hub has grown. Although meeting these checklist items is not a prerequisite to being added to the Hub, we have found that packages which don't conform provide a worse user experience.

First run experience

• [x] The package includes a README which explains how to get started with the package and customise its behaviour
• [x] The README indicates which data warehouses/platforms are expected to work with this package

Dependencies

Dependencies on dbt Core

• [x] The package has set a supported require-dbt-version range in dbt_project.yml. Example: A package which depends on functionality added in dbt Core 1.2 should set its require-dbt-version property to [">=1.2.0", "<2.0.0"].

  Dependencies on other packages defined in packages.yml:

• [x] Dependencies are imported from the dbt Package Hub when available, as opposed to a git installation.
• [x] Dependencies contain the widest possible range of supported versions, to minimise issues in dependency resolution.
• [x] In particular, dependencies are not pinned to a patch version unless there is a known incompatibility.

  Interoperability

• [ ] Optionally let's you override The package does not override dbt Core behaviour in such a way as to impact other dbt resources (models, tests, etc) not provided by the package.
• [x] The package uses the cross-database macros built into dbt Core where available, such as {{ dbt.except() }} and {{ dbt.type_string() }}.
• [x] The package disambiguates its resource names to avoid clashes with nodes that are likely to already exist in a project. For example, packages should not provide a model simply called users.

Versioning

• [x] (Required): The package's git tags validates against the regex defined in version.py
• [x] The package's version follows the guidance of Semantic Versioning 2.0.0. (Note in particular the recommendation for production-ready packages to be version 1.0.0 or above)