Added UnitTests category to test cases from various projects.

As a part of my task MAGN-4263, I have added Category UnitTests.
RevitNodeTests project yet to add these categories.
THere was one build error because of bracket missing in ListTests.cs.

I forgot to build after making changes, sorry for this.
Merge pull request #2291 from riteshchandawar/CategoryUnitTests

Added UnitTests category to test cases from various projects.