Write proper unit tests for deployer

[aTeoke]

Currently, only a fraction of deployer’s functionality is tested. Granted, we won’t be able to test the deployment itself (that will be most likely mocked) but proper argument handling and passing can be tested.
Notes by Emanuel:

• Also, what should be tested is the deployer CLI output and error handling.
• You can mock the outputs of the deployments with some fixtures obtained from failed deployments (just run GCP a few times to get some).
• What I consider the most important is reasonable format of the error descriptions which is presented to the user.
• Also rename the test directory from “__ test__” to “test“