Robot tests of the lib should target the source code instead of the installed lib

We did a testing dojo session using the tests of this lib as a material. I paired with Jérémy Colombet and we thought it would be nicer to test the source code instead of the installed/deployed lib. That way we can run the test after any modification in the source code of the lib without deploying it. On the other hand, we don't have any test left that actually use the deployed lib (we could add one to specifically test that) but we don't think it is the purpose of those test. 

We also removed a duplicated test and slightly reviewed coding style for consistency.