autocomplete test: Add extra pump in new test for new RawAutocomplete delay

Fixes zulip#1289.

Author: chrisbobbe

test/widgets/autocomplete_test.dart

@@ -330,6 +330,10 @@ void main() {
         text: 'some',
         selection: TextSelection(baseOffset: 1, extentOffset: 3)));
       await tester.pump();
+      // Add an extra pump to account for any potential frame delays introduced
+      // by the post frame callback in RawAutocomplete's implementation.
+      await tester.pump();
+
       check(controller.value)
         ..text.equals('some')
         ..selection.equals(