Fix logic for skipping _by_url results in buildbot reults object

The original buildbot results object had this beautific compound if, gift wrapped for posterity in several levels of parenthesis:
  if not self._trace_tag and (len(value_and_display_name_list) > 1 or
      ((self.errors or self.failures) and
       len(value_and_display_name_list) == 1)):
    ... printing logic

I thought I had figured out what it did. Evidently I got it wrong. Oops!

This patch adds more test coverage for the case I missed, and the new tests all pass. So, maybe I got it right this time?

TBR=tonyg
BUG=323382

Review URL: https://codereview.chromium.org/87473003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237310 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237310