cli: implement `test` command

[tiborsimko]

Following up the addition of the reana-client test command to the Python client:

$ reana-client test --help
Usage: reana-client test [OPTIONS]

  Test workflow execution, based on a given Gherkin file.

  Gherkin files can be specified in the reana specification file (reana.yaml),
  or by using the ``-n`` option.

  The ``test`` command allows for testing of a workflow execution, by
  assessing whether it meets certain properties specified in a chosen gherkin
  file.

  Example:     $ reana-client test -w myanalysis -n test_analysis.feature
  $ reana-client test -w myanalysis     $ reana-client test -w myanalysis -n
  test1.feature -n test2.feature

Options:
  -n, --test-files TEXT    Gherkin file for testing properties of a workflow
                           execution. Overrides files in reana.yaml if
                           provided.
  -t, --access-token TEXT  Access token of the current user.
  -w, --workflow TEXT      Name or UUID of the workflow. Overrides value of
                           REANA_WORKON environment variable.
  --help                   Show this message and exit.

We should do the same for the Go implementation of the client.

(But ideally only after the test command processing will be moved to happen on the server-side, like create and validate.)