add tests, refactorize project files, improve sandbox

- add initial ut for rndssucli
- extract libssu-testutils
- refactorize project files
- move SandboxFileEngineHandler from tests to libssu and extend it with ability to work on temporary copy and define its scope of action (this process vs children processes)
- use extended sandbox functionality in existing tests.

Finally I found better to put SandboxFileEngineHandler into libssu - it depends on the constants defined inside libssu so it would be tricky to... Note that I gave the header _p.h suffix, so it is clear it is not part of public API.