chore: Add comprehensive unit tests for Config API

[kapil27]

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes #2885

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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[coveralls]

Pull Request Test Coverage Report for Build 18553777112

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+4.1%) to 56.237%

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Totals
Change from base Build 18529281041:	4.1%
Covered Lines:	1348
Relevant Lines:	2397

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💛 - Coveralls