[SuperEditor][SuperReader] - Can't call `WidgetTester.tap()` on a `SuperReader` due to slivers

[matthew-carroll]

I tried to tap on a SuperReader in a test:

await tester.tap(find.byType(SuperReader));

This produced an error:

══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following assertion was thrown running a test:
The finder "Found 1 widget with type "SuperReader": [
  SuperReader(state: SuperReaderState#7d0cf),
]" (used in a call to "tap()") found an element whose corresponding render object is not a RenderBox
(it is a _RenderSliverHybridStack: "_RenderSliverHybridStack#7a5c0 relayoutBoundary=up30").
Unfortunately "tap()" only supports targeting widgets that correspond to RenderBox objects in the
rendering.

[matthew-carroll]

@knopp can you take a look at how we can support standard WidgetTester interactions after the integration of HybridStack?