emoji_reaction test: Use pumpAndSettle to let overscroll animation finish

These tests scroll a long way, in order to be sure to scroll to the
end of the list.  That means they invoke overscroll.  The details of
how overscroll behaves are changing in an upstream PR:
  flutter/flutter#173849

The new behavior will take more than a single `pump` to reach the
end state where the subsequent checks behave as expected.  These
tests aren't about the overscroll behavior, so just gloss over any
wrinkles by calling `pumpAndSettle`.

Author: MTtankkeo

test/widgets/emoji_reaction_test.dart

@@ -613,7 +613,7 @@ void main() {
 
         // Scroll to the very bottom of the list with a large offset.
         await tester.drag(scrollViewFinder, Offset(0, -500));
-        await tester.pump();
+        await tester.pumpAndSettle();  // let overscroll finish
         // The top edge of the list entries is out of view;
         // the bottom is padded by 8px, the minimum padding, from the bottom
         // edge of the scroll view.
@@ -637,7 +637,7 @@ void main() {
 
         // Scroll to the very bottom of the list with a large offset.
         await tester.drag(scrollViewFinder, Offset(0, -500));
-        await tester.pump();
+        await tester.pumpAndSettle();  // let overscroll finish
         // The top edge of the list entries is out of view;
         // the bottom edge is padded by 10px from the bottom edge of the scroll
         // view, because the view bottom padding is larger than the minimum 8px.