[libcxx] Run u16string tests for gdb pretty printers

As far as I can tell these were just missed out when the tests
were first added. No specific reason they should be skipped.

libcxx/test/libcxx/gdb/gdb_pretty_printer_test.sh.cpp

@@ -659,6 +659,7 @@ int main(int, char**) {
   string_test();
   a_namespace::string_view_test();
 
+  u16string_test();
   u32string_test();
   tuple_test();
   unique_ptr_test();