tests: add unit tests for core and worker controllers

[andoriyaprashant]

Description

Fixes #257

This PR adds comprehensive unit tests for the remaining core controllers under controllers/controller/ and worker controllers under controllers/worker/ to improve test coverage, maintainability, and early detection of regressions.

The following controllers are now covered with unit tests:

Core Controllers:

• cluster_controller.go
• serviceexportconfig_controller.go
• sliceqosconfig_controller.go

Worker Controllers:

• workersliceconfig_controller.go
• workerserviceimport_controller.go
• workerslicegateway_controller.go

Checklist:

• The title of the PR states what changed and the related issues number (used for the release note).
• Does this PR requires documentation updates?
• I've updated documentation as required by this PR.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have tested it for all user roles.
• I have added all the required unit test cases.

[gourishkb]

Hey @andoriyaprashant thanks for the PR, can you please attach the coverprofile output here? I am going over your code changes