sticky_header test: Use 10 items instead of 100

This is still enough to fill more than the viewport, and will be more
helpful for scrolling past an entire sliver when we add more slivers.

Author: gnprice

test/widgets/sticky_header_test.dart

@@ -256,7 +256,7 @@ Future<void> _checkSequence(
     SliverStickyHeaderList(
       headerPlacement: headerPlacement,
       delegate: SliverChildListDelegate(
-        List.generate(100, (i) => buildItem(i)))),
+        List.generate(10, (i) => buildItem(i)))),
   ];
 
   final double anchor;