Azure DevOps pipeline not running xUnit tests

Problem:
Azure DevOps pipeline was not executing XUnit tests

Solution:
The pipeline was running on a machine where no versions of Microsoft Visual Studio were installed.To solve the issue, we had to:

1. add a VisualStudioTestPlatformInstaller task to the pipeline
2. add the xUnit.runner.VisualStudio nuget package reference to our unit test project.