Not all V8 build configs support JS shared memory features. Trying to
create a new shared Isolate on such a config DCHECKs at runtime. Make
the shared Isolate test fixture conditionally initialize the shared
Isolate. Users must explicitly check for support.

Bug: v8:12547
Change-Id: I3df1ce7eb5ae9a3c136f88ea8f44c650cc0408ab
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3687565
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80961}