[Graalvm Test Times] Reduce duplicate verification in java-compute/java-aiplatform

The graalvm presubmits (java-compute and java-aiplatform) are taking ~1 hr to run on each PR. Given the extensive coverage of showcase tests for GAX and the minimal added functionality in the GAPIC layer, we can now streamline our GraalVM monorepo tests to focus on the library's ability to communicate to the service.

This list can be used a reference to determine client behavior that's already covered by showcase tests: https://github.com/googleapis/sdk-platform-java/tree/ea80d1cac8ab8456e764d8d1b2bedeaeebb757d4/showcase/gapic-showcase/src/test/java/com/google/showcase/v1beta1/it