ci: introduce inter-client integration tests

[tiborsimko]

After introducing unit tests (#3), it will be good to introduce more global integration tests that would compare outputs of reana-client-go to the output of the Python reference implementation reana-client for various commands and various switches, in order to make sure which functionality is ported, which functionality is not ported yet, and to make sure that changes in one client and reflected in the other client in time.

We could make a huge list of typical commands and their options, run the two clients as binary blobs against a demo instance, and compare the textual outputs and exit status codes.