Improve 'NestedScrollView and internal scrolling' test to account for all the...

Improve 'NestedScrollView and internal scrolling' test to account for all the inner children layers (#102309)

packages/flutter/test/widgets/nested_scroll_view_test.dart

@@ -599,11 +599,17 @@ void main() {
           return child;
         }
         if (child is ContainerLayer) {
-          final PhysicalModelLayer? candidate = _dfsFindPhysicalLayer(child);
+          Layer? innerChild = child.firstChild;
+          while (innerChild != null) {
+            if (innerChild is ContainerLayer) {
+              final PhysicalModelLayer? candidate = _dfsFindPhysicalLayer(innerChild);
                 if (candidate != null) {
                   return candidate;
                 }
               }
+            innerChild = innerChild.nextSibling;
+          }
+        }
         child = child.nextSibling;
       }
       return null;