RAPI client: Tidy and test WaitForJobCompletion

- Use constants
- Don't sleep if no delay is given
- Mark function as deprecated: it uses polling instead of waiting for changes
  (but the latter needs authentication); it can still be used
- Add unittests

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>