The `brew services list` command was not correctly handling services
that had an error code status.
While the `#zero?` method returns a boolean, the `#nonzero?` method
confusingly returns self or nil. Hence a negated `#zero?` call to check
for a non-zero exit code fixes the error.
While here, `#pid?` method uses a negated `#zero?`, which is not
accurate, as a negative PID value would not be a valid PID. Hence I
changed it to use `#positive?` instead.
The tests for the `#error?` method were marked as needing systemd, but I
saw no obvious reason for that due to how they all use mocked values, so
I removed the systemd requirement.
f969c05b20