arrow-cls: setup for running unit tests

[ivotron]

in the arrow-cls branch, add ability to run tests. At a high-level:

• [ ] copy https://github.com/ceph/ceph/blob/octopus/src/test/cls_sdk/test_cls_sdk.cc to src/cls_sdk_test.cc
• [ ] add test_util.cc with utility functions so that cls_sdk_test.cc runs fine out-of-tree (in the SDK setup)
• [ ] update the builder Dockerfile to include gtest
• [ ] modify the cmake config to add
• [ ] add a new step to the Popper workflow to run the tests