Change the unittest runner to no longer uncondtionally set up a default
platform in the "environment", but to instead make platform set-up part
of the "mixin" framework for test fixtures.

Requires modifying some tests that expect the platform to be available,
and all flag implications resolved, before the mixin constructors run.

We still keep the environment for setting up the process for cppgc. This
process setup can only be done once per process, so it can no longer use
the platform -- that's ok though, the page allocator used by cppgc's
process initialisation doesn't have to be the same as the platform's so
we can just pass in a separate new one.

Change-Id: Ic8ccf39722e8212962c5bba87350c4b304388a7c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3571886Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79820}