The test is flaky due to huge string allocation.

Bug: v8:7093
Change-Id: I2f17eb8b96b569d84a1f5c9549753fd0ab7fee89
Reviewed-on: https://chromium-review.googlesource.com/785351
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49575}