i64 values can be up to 20 characters long (19 + sign), plus we need
one character for the terminating null character. Thus the previous 18
needs to be increased to 21.
Also extend the test to check the longest possible i64 values.

R=ahaas@chromium.org
CC=kozyatinskiy@chromium.org

Bug: v8:8644
Change-Id: Ia9458db162a55dd57b5e8bc7cf7db73c3bab4734
Reviewed-on: https://chromium-review.googlesource.com/c/1404443Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58724}