gnatsd 0.5.6 (new formula)

gnatsd 0.5.6  -> https://nats.io/
Can this test be modified to do something more substantial than e.g. `--version` or `--help`? See `cmake.rb` for an example of an application formula with a good test and `tinyxml2.rb` for an example of a library formula with a good test. Thanks!