Handle output in stderr gracefully when discovering tests

Discovering tests did fail as soon as there is output in ``stderr`` during the discovery process. This happened even when the output is just a ``INFO`` level log. To prevent this from happening, I have changed the ``Project`` model to collect the errors and only raise an exception if no test can be found **and** there are errors in ``stderr``. Otherwise, ``stderr`` will be displayed in a stack trace dialog but allow to continue with the discovered tests.

I added a few tests for the discovery message. I am not quite sure about the format or your preference, so please let me know if there's anything that need to be changed. Also, I am not sure if the ``requirements_test.txt`` is the right way to deal with test-only dependencies. How do you want to handle that?