Add unit testing capabilities and basic tests for check_for_usbdevice

* Add a test directory and create file
  'test/grml2usb_test.py'
* Provide basic test for extract_device_name()
* Extract regex match into separate function for
  better readability and testing options
* Provide pytest.ini configuration to be able to use
  custom markers
* Add symlink in test directory pointing to grml2usb
  for usage with importlib