Replace `v8_unittests.cmx` with `v8_unittests.cml`, thus migrating the
tests to CFv2.

Bug: chromium:1256503
Change-Id: Iab90b406973585a513bce5a1a0884860c7c88e90
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3826050Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Derek Gonyeo <dgonyeo@google.com>
Cr-Commit-Position: refs/heads/main@{#82692}