Implement a testing harness for functional tests

Writing tests is currently clunky and a lot of effort. I propose to develop a testing harness that provides "common functionalities" such as starting a gekkofs server, sending an RPC to it, etc. so that we can execute "scriptable tests". Tests should be automatically found by the infrastructure and should return easy to interpret error codes (e.g. success/failure). It should also be possible to integrate it with CMake.