Bump limit for isolate creation in cctest; the test started crashing recently
on nosnap.debug builds, hence we bump the limit.

Bug: v8:7856
Change-Id: I7c2396c7f112a2ed7fc189f0fa72658e0ed75050
Reviewed-on: https://chromium-review.googlesource.com/1104691Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53816}