Add script to find warnings shared across buildbot history.

Suppressed tests that nonetheless pass are recorded as warnings by vogar.
By finding the tests that are showing up as warnings for all buildbot
setups, we can quickly gather a set of candidates to remove from the
exceptions files.

Test: ran the script
Change-Id: I45e0e460e0077aec239cdd65e187d7d573a0c28f