migrate to scalatest

[xuwei-k]

Pull Request Checklist

• [x] Have you read through the contributor guidelines?
• [x] Have you squashed your commits?
• [x] Have you added copyright headers to new files?
• [x] Have you checked that both Scala and Java APIs are updated?
• [x] Have you updated the documentation for both Scala and Java sections?
• [x] Have you added tests for any changed functionality?

Fixes

Purpose

Background Context

References

[mkurz]

@xuwei-k Conflicts...

[mkurz]

@xuwei-k Can you please once more resolve conflicts and rebase onto master?