Run KAS as part of the test suite
#327853 (closed) introduced a gRPC client to allow the Rails app to communicate with KAS, with all gRPC calls stubbed.
As part of the test suite we should compile and run a "real" KAS instance to verify communication between the two services, similar to how we test against Gitaly.
If possible, avoid setting up KAS for every test, to limit impact to the overall build time.