The attributes tested_items and pass_rate are added for easier display in the
n_way_comparison function.
Bug: 296988808
Test: ./compare_cts_reports.py -r test_result_1.xml -r test_result_2.xml -m n \
-d output_dir
Change-Id: I1dc6bb9c33dfe5be879cbdc13f684bde123e5e71