fix(services/list): correctly handle services with an error code

[jimeh]

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.

---

Resolves this error I was getting while the node_exporter service had a error status:

$ brew services list
Error: Return value: Expected type T::Boolean, got type Integer with value 512
Caller: /opt/homebrew/Library/Homebrew/services/formula_wrapper.rb:277
Definition: /opt/homebrew/Library/Homebrew/services/formula_wrapper.rb:168 (Homebrew::Services::FormulaWrapper#error?)
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11933/lib/types/configuration.rb:296:in `call_validation_error_handler_default'
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11933/lib/types/configuration.rb:303:in `call_validation_error_handler'
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11933/lib/types/private/methods/call_validation.rb:322:in `report_error'
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11933/lib/types/private/methods/call_validation.rb:291:in `validate_call'
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11933/lib/types/private/methods/_methods.rb:277:in `block in _on_method_added'
/opt/homebrew/Library/Homebrew/services/formula_wrapper.rb:277:in `status_symbol'
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11933/lib/types/private/methods/call_validation_2_7.rb:59:in `bind_call'
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11933/lib/types/private/methods/call_validation_2_7.rb:59:in `block in create_validator_method_fast0'
/opt/homebrew/Library/Homebrew/services/formula_wrapper.rb:202:in `to_hash'
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11933/lib/types/private/methods/call_validation_2_7.rb:919:in `bind_call'
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11933/lib/types/private/methods/call_validation_2_7.rb:919:in `block in create_validator_method_medium0'
/opt/homebrew/Library/Homebrew/services/formulae.rb:28:in `map'
/opt/homebrew/Library/Homebrew/services/formulae.rb:28:in `services_list'
/opt/homebrew/Library/Homebrew/services/commands/list.rb:15:in `run'
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11933/lib/types/private/methods/call_validation.rb:282:in `bind_call'
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11933/lib/types/private/methods/call_validation.rb:282:in `validate_call'
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11933/lib/types/private/methods/_methods.rb:277:in `block in _on_method_added'
/opt/homebrew/Library/Homebrew/cmd/services.rb:148:in `run'
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11933/lib/types/private/methods/call_validation.rb:282:in `bind_call'
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11933/lib/types/private/methods/call_validation.rb:282:in `validate_call'
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11933/lib/types/private/methods/_methods.rb:277:in `block in _on_method_added'
/opt/homebrew/Library/Homebrew/brew.rb:95:in `<main>'
Please report this issue:
  https://docs.brew.sh/Troubleshooting

---

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---

The one failure I got locally with brew tests was from test/dev-cmd/tap-new_spec.rb which seems very unrelated to my fix here:

Failures:

  1) Homebrew::DevCmd::TapNew initializes a new tap with a README file and GitHub Actions CI
     Failure/Error:
       expect { brew "tap-new", "homebrew/foo", "--verbose" }
         .to be_a_success
         .and output(%r{homebrew/foo}).to_stdout
         .and not_to_output.to_stderr

          expected #<Proc:0x00000001369baaf0 /opt/homebrew/Library/Homebrew/test/dev-cmd/tap-new_spec.rb:13> to be a success

       ...and:

          expected block to not output to stderr, but output "error: cannot run gpg: No such file or directory\nerror: gpg failed to sign the data:\n(no gpg output)\nfatal: failed to write commit object\nError: Failure while executing; `git commit -m Create\\ homebrew/foo\\ tap` exited with 128.\n"
     # ./test/dev-cmd/tap-new_spec.rb:13:in `block (2 levels) in <top (required)>'
     # ./test/support/helper/spec/shared_context/integration_test.rb:50:in `block (2 levels) in <top (required)>'

[botantony]

Cool, thank you!

Do not worry about tap-new error, it is related to your git configuration

[Bo98]

Thanks!