chore: authenticate in NMS prior to making other API calls

[gruyaume]

Description

Now that authentication has been added to the NMS K8s charm, the tests here need to authenticate prior to making other API calls (creating network slices, and subscribers).

This PR Depends on the changes in the NMS K8s charm being merged first:

• feat: add authentication support sdcore-nms-k8s-operator#359

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have bumped the version of the library