Refactor unit tests

What does this MR do?

Refactors the unit tests:

• Modifies some unit tests to call t.Setenv rather than os.Setenv. The former provides inbuilt cleanup and restores the previous value of the env var once the test has concluded. See !42 (comment 1212968032)
• Uses table tests consistently.
• Uses testify consistently.

What are the relevant issue numbers?

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
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  • Job definition example
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• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer