Expose ArgumentParserTestHelpers

[miggs597]

It would be nice to have access to ArgumentParserTestHelpers, this would allow SwiftPM to use AssertHelp to prevent regression.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

We aren't ready to maintain that module as public API yet — for testing a command's help output, you can use the static helpMessage(for:) on any ParsableCommand type.