Test coverage cleanups

[freakboy3742]

• Restores 100% core test coverage for DateInput and TimeInput (caused by a missing code path in the deprecation shim)
• Restores 100% cocoa test coverage for Button (caused by the recent change to interpret default widget height as NONE, rather than 0)
• Ports the toga.platform tests to pytest. Not sure why, but the introduction of the NotImplmented factory shim in Improve error message when a platform doesn't support a widget. #1992 caused local pytest runs to fail for me. Tox runs were fine; but since we needed to update the tests anyway, now was as good a time as any. This also corrects a minor issue with the definition of current_platform, and gets the module to 100% test coverage.

PR Checklist:

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

[freakboy3742]

Closed in favor of #2005