Migrate tests to JUnit5

[strangelookingnerd]

This PR aims to migrate all tests to JUnit5. Changes include:

• Migrate annotations
• Remove public visibility of test classes and methods

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• Please describe what you did
• Link to relevant issues in GitHub or in Jenkins JIRA
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• Did you provide a test-case? That demonstrates feature works or fixes the issue.

[github-actions]

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[strangelookingnerd]

@jenkinsci/onepassword-secrets-plugin-developers Kindly requesting a review.