Add a tool which compares failed tests

Given two test_result.xml's (A and B), this script lists all failed
tests in A, and shows result of the same test in B. This is usually
used for regression detection or device stability analysis.

Bug: 79442777
Test: ./compare_failed_tests.py test_result_a.xml test_result_b.xml
Change-Id: I7778b5d748daccdeadabe6a539e1ce9622e535c5