compose_box test: Use a layout similar to the message list page

This gives more vertical space above the compose box.  This will provide
enough space for the autocomplete to appear without overflowing when we
add tests for it.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

test/widgets/compose_box_test.dart

@@ -51,7 +51,13 @@ void main() {
 
     final controllerKey = GlobalKey<ComposeBoxController>();
     await tester.pumpWidget(TestZulipApp(accountId: eg.selfAccount.id,
-      child: ComposeBox(controllerKey: controllerKey, narrow: narrow)));
+      child: Column(
+        // This positions the compose box at the bottom of the screen,
+        // simulating the layout of the message list page.
+        children: [
+          const Expanded(child: SizedBox.expand()),
+          ComposeBox(controllerKey: controllerKey, narrow: narrow),
+        ])));
     await tester.pumpAndSettle();
 
     return controllerKey;