Fix for Issue #239 - Created Unit Tests for Install-ChocolateyDesktopLink and added Synopsis

In line with the changes that were made in #238 for Issue #235, I have added some unit tests to the Install-ChocolateyDesktopLink Helper Method, as well as added in a Synopsis, based on the instructions that are located here: https://github.com/chocolatey/chocolatey/wiki/HelpersInstallChocolateyDesktopLink

Thanks

Gary