Use built-in JUnit Platform support in Gradle 4.6
Use built-in JUnit Platform support in Gradle 4.6

Also remove test runtime dependencies needed for @API report -- the
report generator moved to the documentation project. Due to that removal
Groovy was no longer on the class-path. It is now required explicitly by
`groovy-jsr223` re-using the Groovy version that drives Gradle.

junit-team/junit-lambda,sbrannen/junit-lambda