Increase test coverage for authprovider

[binbin-li]

What would you like to be added?

Add more unit tests to increase test coverage for authProvider. To be specific, add more tests to authprovider.go, authproviderfactory.go and k8secret_authprovider.go under https://github.com/deislabs/ratify/tree/main/pkg/common/oras/authprovider.

Test coverage can be checked in: https://app.codecov.io/gh/deislabs/ratify/tree/main/pkg/common/oras/authprovider

Anything else you would like to add?

No response

Are you willing to submit PRs to contribute to this feature?

• Yes, I am willing to implement it.