A set of integration tests for backend

We really need one. Unit tests are good to have,
but they really don't catch that many bugs in a statically-typed
language like Go. Integration tests, on the other hand,
catch quite a lot.

The best case scenario is that we get a statically-compiled
binary, AdGuardHome.test, which we can send
to the target machine, run it, and get test results, coverage
reports, perhaps even CPU and memory benchmark
reports. I have some experience making such stuff,
but not on this scale.