tests: enrich test suite

[tiborsimko]

The current test suite only has one example for testing version import, see tests/test_version.py.

We should start introducing tests as the feature set grows, see https://github.com/cernopendata/cernopendata-client/issues/19#issuecomment-686983360

The tests to introduce are of diverse nature:

• "true unit tests" for helper functions such as validate_recid() and validate_server();

• "integration tests" or "regression tests" for CLI commands such as cernopendata-client get-file-locations --title '/DoubleElectron/Run2012B-v1/RAW'

For inspiration, see reana-client or reana-dev test suite.

[tiborsimko]

Closed by #35 and #34.