msglist: Drop easing curve in scroll-to-bottom

Fundamentally an easing curve like this relies on knowing in advance
how far the animation is going to end up going.  When we start
letting the message list scroll from the middle of history, it'll no
longer be possible to know that.

Switch instead to a behavior that can be based on only what's happened
so far, not on a prediction of the future.  In the future, if we want,
we could get fancy and make the speed change over time; but to start,
keep it simple, and just move at the same speed from start to finish.

Author: gnprice

lib/widgets/message_list.dart

@@ -703,7 +703,7 @@ class ScrollToBottomButton extends StatelessWidget {
     return scrollController.animateTo(
       target,
       duration: Duration(milliseconds: durationMs),
-      curve: Curves.ease);
+      curve: Curves.linear);
   }
 
   @override

test/widgets/message_list_test.dart

@@ -511,8 +511,8 @@ void main() {
       check(isButtonVisible(tester)).equals(false);
     });
 
-    testWidgets('scrolls at reasonable speed', (tester) async {
-      const referenceSpeed = 8000.0;
+    testWidgets('scrolls at reasonable, constant speed', (tester) async {
+      const maxSpeed = 8000.0;
       const distance = 40000.0;
       await setupMessageListPage(tester, messageCount: 1000);
       final controller = findMessageListScrollController(tester)!;
@@ -536,9 +536,10 @@ void main() {
         pos = controller.position.pixels;
         log.add(pos - lastPos);
       }
-      // Check the main question: the speed stayed in range throughout.
-      const maxSpeed = 2 * referenceSpeed;
-      check(log).every((it) => it..isGreaterThan(0)..isLessThan(maxSpeed));
+      // Check the main question: the speed was as expected throughout.
+      check(log.slice(0, log.length-1)).every((it) => it.equals(maxSpeed));
+      check(log).last..isGreaterThan(0)..isLessOrEqual(maxSpeed);
+
       // Also check the test's assumptions: the scroll reached the end…
       check(pos).equals(0);
       // … and scrolled far enough to effectively test the max speed.