Rewrite groovy tests into kotlin

[jimirocks]

Preferred is Kotlin, howeve java is also acceptable.

[glunkad]

Hey @jimirocks , I'm interested in contributing to this issue, so before I start working it, would you mind sparing your time explaining what the issue is about and pointing me to some resources to get started.

[jimirocks]

Hi @9gl also @knotekt will start working on this, which is OKish since it's not small task. The basic idea:

• prefer Kotlin + junit5 + strikt.io + mockk
• we can do it per partes - no need for big bang, let's do it one after other, even we can create several pull requests
• there are already few kotlin tests in place, so the boilerplate is there
• the finite state is to remove groovy and spockframework dependency

SIDE NOTE: I am on Vacation 12.-20.8. so in the mean time @xzavrel could be available to review