Add GZIP utility to testing package

As discussed in an earlier call, we should add the ability to GZIP content to the testing package to help us accelerate development of tests. We could add this under org.eclipsefoundation.testing.helpers.SerializationHelpers, as there may be other serialization helpers in the future. This way we can set up for the future rather than coding ourselves into a corner by naming it for GZIP functionality.