This package can be utilized to make use of Jest as a testing framework for SharePoint framework solution.
It explains how test can be written for SPFx development.
It also provide shows how Entity merging using PnP Js can be implmented.
Usage : Step 1) Download the package
Step 2) Run the below commands in Windows powershell.
For more information on how Test driven development approach can be utilized in SharePoint Framework solution, refer the below article