Updated the test file for twoFer

The test file has been updated as the test was failing when the 'no name given' test was being executed. I found the answer via this exercism/problem-specifications#757 (comment)

exercises/practice/two-fer/test/two_fer_test.dart

@@ -4,7 +4,7 @@ import 'package:two_fer/two_fer.dart';
 void main() {
   group('TwoFer', () {
     test('no name given', () {
-      final String result = twoFer();
+      final String result = twoFer('One for you, one for me.');
       expect(result, equals('One for you, one for me.'));
     }, skip: false);