test: cover Environments sharing an embedder-owned isolate

Signed-off-by: Shelley Vohr <shelley.vohr@gmail.com>
PR-URL: #66239
Refs: #65977
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: codebytere