DUI: tests for conveters.

Last PR, but not least ;)
This is related to a [previous pull request](https://github.com/Benglin/Dynamo/pull/154).

Reviewers
@Benglin ,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5282](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5282) DUI: Add unit tests for all Sitrus converters