Convert not-implemented to use warnings

[freakboy3742]

This is a set of changes that came out of the implementation of MapView in #2379.

Toga has provided a not_implemented() helper for a long time; however, it's been implemented as a simple print statement. This means it cannot be easily monitored or silences, and repeated calls to the method result in repeated messages in the console (which can be overwhelming).

This converts not_implemented() to raise a NotImplementedWarning. The slightly odd warn() class method is needed because pytest.warns() requires warnings to accept a single argument (the warning message) if they're uncaught and reraised.

To ensure this new feature is exercised, WebView.on_webview_load handler has been modified to raise a warning if the backend doesn't support the handler (as is the case with Android). MapView will have analogous handling for on_select events on the map.

To ensure the warnings are handled, this PR also promotes warnings to full errors in the core tests. This was already being done on the testbed, but the implementation has been modified in this PR to ignore NotImplementedWarnings. This revealed a number of warnings that the core test suite wasn't handling; tests have been updated to reflect this. Amongst these warnings were warnings on Python 3.12 about there being no current event loop; the app tests that instantiate toga.App have all been modified to reference the event_loop fixture to ensure an event loop exists.

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