[Tests] increase coverage

See tc39/test262#4038

test/tests.js

@@ -57,6 +57,24 @@ module.exports = function (groupBy, t) {
  'thisArg and callback arguments are as expected'
  );
 
+ st.test('strings', function (s2t) {
+ var obj = groupBy('abcd💩', function (char) {
+ return char < 'c' ? 'before' : 'after';
+ });
+
+ s2t.deepEqual(
+ obj,
+ {
+ __proto__: null,
+ before: ['a', 'b'],
+ after: ['c', 'd', '💩']
+ },
+ 'grouping a string works as expected'
+ );
+
+ s2t.end();
+ });
+
  st.end();
  });
 };