Add unit and functional tests of the SAL client

[TOFarmer]

Description of work

Add unit and functional tests of the SAL client using pytest.

All tests are documented using GIVEN-WHEN-THEN, and I would recommend this approach is used for all future tests.

All tests which are marked as expected fails have corresponding issues:

• 66 (test_authenticate_not_required)

• 67 (test_connect_to_new_host)

• 68 (test_copy)

• 69 (test_connect_to_host_no_response)

Fixes

Fixes #65

To test

• [x] Run tests to confirm that they pass or are expected failures.

Reviewer checklist

• [ ] The author suggested tests are successful.
• [ ] Relevant documentation (user and developer) has been added, and is clear and complete.
• [x] Suitable unit tests have been added. If unit tests are not included in this MR, please comment as to why and create an issue to address this.