Fixes RegressionTest Fixture

- So far RegressionTest Fixture was not running properly because
SetupRevitRegressionTests() was throwing exception and hence the status
was Test
- RegressionTest is now derived from SystemTest
- SetUp() must be called after calling SwapCurrentModel so that
RevitDynamoModel initializes DocumentManager with the current active
revit document.
Merge pull request #3299 from Conceptual-Design/master

Fixes RegressionTest Fixture