Also mark another slow tests as SLOW.

TBR=jgruber@chromium.org

Bug: v8:7783
Change-Id: I69a8ac82e7898fa3b374c5b66a441f040d241413
Reviewed-on: https://chromium-review.googlesource.com/1069093Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53306}