[FEATURE] Add tests for Kerberos Auth

[derek-ho]

Is your feature request related to a problem?
Kerberos auth has no tests
What solution would you like?
Tests should be added to verify the behavior of kerberos auth
What alternatives have you considered?
None
Do you have any additional context?
Since we officially support kerberos auth, we should add tests to verify that we do in fact support it, and that the behavior is correct. This is a first step to implementing kerberos support for OpenSearch Dashboards, since that is relying on the backend functioning correctly.

[stephen-crawford]

[Triage] Hi @derek-ho thank you for filing this issue. You are definitely correct that we should have tests for all the features. Going to mark as triaged.