Create NUnitTest abstract class to expose NUnit related functionality

[alelom]

Description:

There are a few functions that NUnit exposes, like OneTimeSetup, and TearDown that we can leverage to expose certain functionality that all BHoM NUnit tests must have. For example:

• Loading required assemblies (not all assemblies): see this example.
• Exposing information logged via the Engine.Base.Compute.Record* methods after running tests: example.

This functionality can be stored in a base class to expose BHoM NUnitTest abstract class.