Fix failedOnce for multiple tests in sheriff-o-matic

If there are multiple tests in a failure group, but each of the tests
has only failed once, we'd incorrectly put this in the reliable failures
category. Instead, evaluate each test independently for failedOnceness.

Also, rename ct-failure-group-tests to ct-sheriff-failure-group-tests.

NOTRY=true
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/586263002

git-svn-id: svn://svn.chromium.org/blink/trunk@182466 bbb929c8-8fbe-4397-9dbb-9b2b20218538