Sheriff-o-matic: Display snoozed failure groups in a separate stream

Adds a category to ct-failure-group and enables ct-failure-stream to filter
to a single category.

Ports the previously broken ct-failure-stream-tests to mocha in order to
add new tests.

NOTRY=true
BUG=400397,399967, 400387

Review URL: https://codereview.chromium.org/455263002

git-svn-id: svn://svn.chromium.org/blink/trunk@180314 bbb929c8-8fbe-4397-9dbb-9b2b20218538