Change the unittests Isolate mixin to create one Isolate per test,
rather than one per test suite. We usually run these tests independently
in separate processes anyway, so this shouldn't affect normal test
execution, but it will avoid Isolate state leaking across tests when
running the unittests binary directly.

Take this opportunity to also clean up the mixins, changing counter
initialization and forcing pointer compression into template traits.

Bug: v8:10142
Change-Id: If92046f9c6f2056252d099faed04d97844ef7319
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2143818Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67110}