Add Mocked Tests: Image Actions

[loosla]

This PR is adding mocked tests for image_actions.get, image_actions.list, image_actions.post

Related to #211

[andrewsomething]

We may want to fix Makefile: PYLINT_ARGS is empty for make lint-tests.
Not sure what the intention is: should we remove PYLINT_ARGS or assign the same value as in github actions - https://github.com/digitalocean/pydo/blob/main/.github/workflows/pr.yaml#L40

We set PYLINT_ARGS="--disable=fixme" in the GitHub workflow so that TODO comments do not blocker pull requests as we have a number of them in the code base. When run locally, it may be useful to quickly find where we have TODO comments that still need to be addressed. We pass the empty PYLINT_ARGS in the Makefile so that it is passed through in the environment. If you want to disable the warnings locally, you can run:

PYLINT_ARGS="--disable=fixme" make lint-tests

[andrewsomething]

👍 LGTM!