Add CI

[drewbrew]

1. Copy the test apps from https://github.com/beeware/briefcase-macOS-Xcode-template/pull/12 and strip the non-toga bits
2. Copy the CI from briefcase and trim out the non-Android parts
3. Modify the toga pyproject.toml to strip out the non-Android parts

PR Checklist:

• [x] All new features have been tested
• [ ] All new features have been documented
• [x] I have read the CONTRIBUTING.md file
• [x] I will abide by the code of conduct

[drewbrew]

@freakboy3742 I think this one is good to go but we can't test until any actions get in main