gradle-extra-configurations-plugin | Gradle plugin introducing a provided dependency | Plugin library

I am migrating a Maven library project to Gradle. The original project also has optional dependencies. I use the java-library plugin but moving the formerly optional dependencies to implementation results in runtime dependencies instead of compile. So I tried the gradle feature variants which results in the right dependencies in the pom.xml. But doing so results is failing test compile as the dependencies of the feature variant are missing on the test compile classpath!

Here is my current setup in build.gradle: