Fix ct-failure-analyzer's failure comparator and test

With the introduction of CTFailureGroup, we no longer pass raw
arrays into _failureListComparator. Teach that function (and the
tests) to use the failures array on that object instead of expecting
the array itself.

R=ojan@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/491693002

git-svn-id: svn://svn.chromium.org/blink/trunk@180612 bbb929c8-8fbe-4397-9dbb-9b2b20218538