... flag-definitions-unittest.

We should setup the isolate to get the correct Flag_*.
This CL fixes the breakage in https://ci.chromium.org/p/v8/g/ports/console including https://ci.chromium.org/p/v8/builders/ci/V8%20Linux%20-%20arm%20-%20sim%20-%20lite and https://ci.chromium.org/p/v8/builders/ci/V8%20Linux%20-%20arm%20-%20sim%20-%20lite%20-%20debug etc.

Bug: v8:12781
Change-Id: Ifca84c02f8458f0a906948f4a7e0f999af429abf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3697865Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: 王澳 <wangao.james@bytedance.com>
Cr-Commit-Position: refs/heads/main@{#81069}