chore(auth): Completed unit tests for AuthenticatorViewModel's signIn region

[edisooon]

• PR conforms to Pull Request guidelines.

Issue #, if available:

Description of changes:
Added missing test cases in AuthenticatorViewModelTest.kt for when AuthSignInStep is one of the following:

• CONFIRM_SIGN_IN_WITH_SMS_MFA_CODE
• CONFIRM_SIGN_IN_WITH_CUSTOM_CHALLENGE
• CONFIRM_SIGN_IN_WITH_NEW_PASSWORD
• CONFIRM_SIGN_UP

How did you test these changes?
(Please add a line here how the changes were tested)

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Ensure commit message has the appropriate scope (e.g fix(liveness): message, fix(authenticator): message, fix(all): message)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

The base branch was changed.