SpringBoot Enable debug option for Kotlin in Gradle Plugin tests

Hi,

this PR enables GradleRunner.withDebug(true) for Kotlin tests as https://github.com/gradle/gradle/issues/6862 seems to be fixed by now.

https://github.com/gradle/gradle/issues/14125 on the other hand still seems to be broken, so I updated the comment accordingly.

Cheers, Christoph

Comment From: wilkinsona

Thanks very much, @dreis2211.