Add Unit and Integration Tests Templates

[PlagueHO]

A set of Unit and Integration Test template files have been developed and are available as part of the xNetworking resource (https://github.com/PlagueHO/xNetworking/tree/dev/Templates). I'm just waiting on this PR for these changes to be moved into the upstream PowerShell repo.

These test templates use the *-TestEnvironment functions in DSCResource.Tests. They also automatically download or update DSCRsource.Tests using Git.exe to the root of the DSC resource being tested.

Ideally these Test templates could be added to this repo so that they are available to all DSC resources. The documentation would also need to be updated to instruct on the use of these Test Templates.

This change was initially discussed here.

Also, this change should also incorporate adding a .gitignore file to the DSCResource.Template folder containing DSCResources.Tests to ensure that the DSCResources.Tests folder is never accidentally added to a DSC Resource.

I've actually made the changes above to my fork of this repo and will submit a PR for this if this is agreeable?

Cheers!