Gradle 4.6 provides built-in support for the JUnit Platform within the
standard `test` task.

This commit configures a custom `testJUnitJupiter` test task for
executing JUnit Jupiter tests directly on the JUnit Platform instead of
indirectly on JUnit 4 via @RunWith(JUnitPlatform.class).

This switch provides for better integration with Gradle's test reporting
and paves the way for a possible transition to the JUnit Platform in the
future.

Issue: SPR-16672