Increase test coverage

What does this MR do?

In this MR, we added more unit tests to increase the test coverage for different files in spotbugs.

Before

➜  spotbugs git:(master) go test -cover instance/instance.go instance/instance_test.go
PASS | command-line-arguments 0.150s
COVER| 52.6% [#####_____]

After

➜  spotbugs git:(add_test) go test -cover instance/instance.go instance/instance_test.go
PASS | command-line-arguments 0.146s
COVER| 86.8% [########__]

What are the relevant issue numbers?

gitlab-org/gitlab#214982 (closed)

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
  • Auto-DevOps template (also in CE)
  • Job definition example
  • CI Templates (to be removed)
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer