The test takes several minutes, because the {slice} call does thousands
of runtime calls, which again call {ValidateElements} for every single
added element (in debug mode).
Hence this CL skips the test in the slow_path variant.

R=leszeks@chromium.org

Change-Id: I2fbaaf32809ecb34de1f563f34bd65ce8b7ab238
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2237628Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68265}