The test was reading from undefined stack addresses instead of creating
a string filled with characters that look like a new space pointer.

Bug: v8:6953
Change-Id: I2c0a9034076012746bd70325a4f21c63f4c264fa
Reviewed-on: https://chromium-review.googlesource.com/725322Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48668}