Fix test in sheriff-o-matic.

I broke this in my last commit. The category was failedOnce instead
of default for this test, which meant the failure group didn't get
stamped out. Instead, make it a regular failure so that the category
becomes default.

TBR=leviw@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/598453006

git-svn-id: svn://svn.chromium.org/blink/trunk@182561 bbb929c8-8fbe-4397-9dbb-9b2b20218538