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

Same reasoning as in 0bfa79d.

This new test was merged in zulip#1266 / 4942f25. At the time, the
breaking change in upstream's RawAutocomplete had been reverted,
because it broke some Google tests:
  flutter/flutter#161666
and it had not yet relanded, which is why our CI passed. But the
reland happened, in
  flutter/flutter#161695
, so our test broke; this fixes it.

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(