Merge of http://go/wvgerrit/74965
It isn't always obvious which changes introduced test failures or fixes,
and similarly, it's unideal to have to manually run the tests for HEAD
and with a new change to determine the test diff, so this script runs it
for the last n changes, and diffs the test results by comparing the test
runs. It doesn't handle flaky tests, partly because I don't think it
can.
Change-Id: I5380abcb2f5bcadf172dba3c1270e4258e0c8a82
