msg_list: Use faster fade-out for mark-as-read animations

Fixes part of zulip#613

Author: Khader-1

lib/widgets/message_list.dart

@@ -635,7 +635,9 @@ class _AnimatedUnread extends StatelessWidget {
               opacity: isRead ? 0 : 1,
               // Web uses 2s and 0.3s durations, and a CSS ease-out curve.
               // See zulip:web/styles/message_row.css .
-              duration: Duration(milliseconds: isRead ? 2000 : 300),
+              // As of now we wait for the ack from the server
+              // so we're going with 0.1s as soon as the ack is received.
+              duration: Duration(milliseconds: isRead ? 100 : 300),
               curve: Curves.easeOut,
               child: child);
   }

test/widgets/message_list_test.dart

@@ -657,7 +657,9 @@ void main() {
         ..status.equals(AnimationStatus.dismissed);
 
       final frames = await tester.pumpAndSettle();
-      check(frames).isGreaterThan(1);
+      // equals 1 when animation duration is 100ms
+      // and increases as duration does.
+      check(frames).isGreaterOrEqual(1);
       check(getAnimation(tester, message.id))
         ..value.equals(0.0)
         ..status.equals(AnimationStatus.completed);