Supporting JUnit 5 and back port the Kotlin Test Sever

[stevehu]

@logi has a gist with a Kotlin Test Server that hides a lot of details in server communication. We can backport to Java once we move to JUnit 5.

https://gist.github.com/logi/acefdd8fb0e60979b755dd24b095754b

[stevehu]

Supporting JUnit 5 and back port the Kotlin Test Sever

[stevehu]

This has been implemented in the OpenAPI Kotlin Generator.