Add a test harness for running the CPython test suite.

[freakboy3742]

Briefcase can now run test suites; this adds a testbed project that can be used to run the CPython test suite.

It's not ready for merging because:

1. The test suite currently crashes on the asyncore tests
2. The support package is an explicit versioned reference. This won't work for tagged releases, or when the Python version changes. We may need to consider modifying Briefcase to allow for command-line overrides of support_package

PR Checklist:

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