Better implementation of test_OpenModelica for Windows users

This pull request has following changes:
- use windows environment variables to get the path to omc
- added status. and request. to ignored files in _setTemporaryDirectories()
- added a unittest for test_OpenModelica()

Additionally there are changes in the test_development_regresstiontest.py:
- checkHTML = False to allow running the unittest without having tidylib.