Report execution order ambiguities in tests

[alice-i-cecile]

What problem does this solve or what need does it fill?

System execution order ambiguities are particularly dangerous in tests, as they lead to flakiness.

The ReportExecutionOrderAmbiguities resource however does not report its results when run inside of a test using manual App.update calls.

What solution would you like?

Create a method on both App and Stage (post-stageless, just Schedule) that fetches execution order ambiguities and stores them in an object for inspection.

Then, use dbg!(app.execution_order_ambiguities()) in your tests when ambiguous execution order is suspected.

What alternative(s) have you considered?

Allow users to forbid execution order ambiguities and panic if any (local) ambiguities are detected.

This would also be useful, but not quite as flexible for this use case.

Additional context

Any PR attempting this should be based off of #2766 #4299, which massively improves this system.