Migrate to test suites Gradle plugin

[bgalek]

Should this plugin switch to Gradle test suites?

https://docs.gradle.org/current/userguide/jvm_test_suite_plugin.html#configure_dependencies_of_a_test_suite_to_reference_project_outputs

[pmendelski]

Definitely it should.

I hoped to see it mature from incubation first (from the same docs: The JVM Test Suite plugin is an incubating API and is subject to change in a future release.) but am willing to merge a change :)

The minimal supported gradle version in gradle-integration-test-plugin was recently upgraded so I it should be easier now.