Remove the testsets plugin, use native Gradle

[binkley]

The very excellent unbroken-dome testsets plugin did the Gradle build configuration for us. With Gradle 8, it has stopped working, and held back this project from upgrading Gradle.

References:

• https://github.com/binkley/modern-java-practices/issues/358
• https://github.com/unbroken-dome/gradle-testsets-plugin/issues/131
• https://docs.gradle.org/current/userguide/java_testing.html#sec:configuring_java_integration_tests