How to have tests for both true and false here?

See the lines you just deleted from the test:

	reconciler.CancelHealthCheckOnNewRevision = true
	t.Cleanup(func() { reconciler.CancelHealthCheckOnNewRevision = false })

See the boolean moving from the reconciler to the SetupWithManager options

Ah right, the test framework we have in place does not cope with this, you can not rebuild the controller once it has started.

Maybe a separate package like we did before?

Or maybe test only true since ideally we would not have this feature gate. If this works well we plan to make it opt-out later, and possibly even ignore it with a warning after even more time.

Yeah enabling it in tests should be Ok. The e2e test suite has the default gate state, so we cover both cases.