Check that each test in `luatest` test-suite contains a brief description

[CuriousGeorgiy]

Now that we use tarantool/luatest framework for testing, our tested are structured into separate functions: I believe it would be useful to enforce adding a brief description stating what is tested and what is expected, e.g.
https://github.com/tarantool/tarantool/blob/2971f691b6eaf38646901496ea9cbde10c4d7f83/test/box-luatest/graceful_shutdown_test.lua#L232-L235