avoid mrb_string_value_cstr() to copy string between mrb_states in tests

tasks/mrbgems_test.rake

@@ -93,8 +93,7 @@ MRuby.each_target do
             f.puts %Q[    val2 = mrb_ary_shift(mrb2, ary2);]
             f.puts %Q[    ]
             f.puts %Q[    while (mrb_test(val2)) {]
-            f.puts %Q[      char *str = mrb_string_value_cstr(mrb2, &val2);]
-            f.puts %Q[      mrb_ary_push(mrb, ary1, mrb_str_new_cstr(mrb, str));]
+            f.puts %Q[      mrb_ary_push(mrb, ary1, mrb_str_new(mrb, RSTRING_PTR(val2), RSTRING_LEN(val2)));]
             f.puts %Q[      val2 = mrb_ary_shift(mrb2, ary2);]
             f.puts %Q[    }]
             f.puts %Q[  }]