Refactor RevitIntegrationTests

In order to make possible the running of system tests on Revit (.dyn-based) (as in http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5124), we need to move the SystemTestBase class to the RevitTestServices library. During this move it made sense to undertake several refactorings which are long overdue.

- DynamoRevitTests is renamed to RevitSystemTests. The dll name is updated to match. DynamoRevitTester.dll will now be RevitSystemTests.dll.
- Dynamo.Tests namepsace is replaced on all Revit integration tests with RevitSystemTests and the proper sub-namespace to match the file's folder. 
- An additional class, SystemTest, is added to act as the base class for all Revit system tasks.
- Setup method for the SystemTestBase class now calls a virtual method SetupPaths, where additional path setup specific to a set of system tests is conducted. For example, in the core Revit system tests this method is where we setup the samples path, the default models path, etc.

PTAL:
- [x] @Steell 

Requires Merge:
- [ ] Revit2015