compose test: Set up compose box using enterContent

So the test is more realistic.  A side effect of this is that the
content input text field is focused afterwards.

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

test/widgets/compose_box_test.dart

@@ -585,7 +585,7 @@ void main() {
       // (When we check that the send button looks disabled, it should be because
       // the file is uploading, not a pre-existing reason.)
       await enterTopic(tester, narrow: narrow, topic: 'some topic');
-      controller!.content.value = const TextEditingValue(text: 'see image: ');
+      await enterContent(tester, 'see image: ');
       await tester.pump();
     }