Pester
Pester is the ubiquitous test and mock framework for PowerShell.

Pester GitHub Repo: https://github.com/pester/Pester

To read the Pester Wiki locally, you can clone the wiki to your computer with:

git clone https://github.com/pester/Pester.wiki.git

To install the latest version of Pester run the following command from an Administrative PowerShell prompt:
Install-Module -Name Pester -Force -SkipPublisherCheck