Minor cleanups to flakytests.py

-Pass -f instead of setting EDITOR.
-Put a message at the top of FlakyTests saying it's autogenerated.
-Add a test that goes through the upload codepath.
-Have ExecutiveMock log calls to run_and_throw_if_fail.

Review URL: https://codereview.chromium.org/312103005

git-svn-id: svn://svn.chromium.org/blink/trunk@175821 bbb929c8-8fbe-4397-9dbb-9b2b20218538