test (config) : Add some unit tests for config settings

[rohanKanojia]

Description

Relates to: #3832, PR #4451

Extracted some unit tests out of #4451 as I needed them while working on #3832

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change
• Chore (non-breaking change which doesn't affect codebase;
  test, version modification, documentation, etc.)

Proposed changes

These unit tests verify these scenarios for config:

• when we try to set invalid key, throw error
• whether default key values are as expected
• whether we're able to override default values by providing new value

Testing

N/A

Contribution Checklist

• I Keep It Small and Simple: The smaller the PR is, the easier it is to review and have it merged
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Which platform have you tested the code changes on?
  • Linux
  • Windows
  • MacOS

[openshift-ci]

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[openshift-ci]

@rohanKanojia: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/integration-crc	1789aeb	link	true	/test integration-crc
ci/prow/e2e-crc	1789aeb	link	true	/test e2e-crc

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