util: Add a simple junit test for the gem5 ops java wrapper.

util: Add a simple junit test for the gem5 ops java wrapper.

If possible, this gets built into the jar which also holds the wrapper.
It can be run on its own through its own main function, or through the
junit mechanism directly.

Change-Id: Iaec7f6deb26bbad75da027c71edca50ef25697cf
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28181
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabe.black@gmail.com>