Fix for broken NUnit reference in RevitTestServices

This relates to a thread between Ritesh, Monika and Neal regarding a ReSharper "Run All Tests" error message that displayed a NUnit.framework.dll "File Not Found" error message. The correct path for it is the GitHub Dynamo path rather than the many other potential locations for the file.

#### Reviewers:
* [ ] @riteshchandawar 
* [ ] @monikaprabhu 
* [ ] @ikeough