Introduce real assert for tests

[Gerold103]

Lua assert() won't kill the process, it only throws an error. For testing it might be better to have a real assert which would abort the process when fails. We could put it into vshard.util.assert() or something alike. In tests it would be redefined to call C function abort() (in test/instances/storage.lua and router.lua). Then we would more reliably catch all asserts which now might go unnoticed, especially when fail inside background services. By default we would keep it equal to Lua assert().