Tests: Added several tests for RC format
Added a test for rc2po converter.
Tests: Several cleanups in RC tests

* Move tests to a class,
* Use helper method to avoid rewriting the code to write and parse a
  dummy file,
* Rewrote code to use explicit assertions instead of iterating in a loop.
Tests: Add method to check RC roundtrip conversion

Still not used, but already available for future use. This method is
now format agnostic, so moving it to a superclass in order to reuse
it in other test classes is advisable.