test: adapt test-v8-flags to V8 9.3

%_IsSmi was removed. %IsSmi can be used instead.

Refs: https://github.com/v8/v8/commit/863a2d6c244be84b54b4903711d86c349b3c0767

PR-URL: https://github.com/nodejs/node/pull/39469
Backport-PR-URL: https://github.com/nodejs/node/pull/39947


Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>