The existing version for paged spaces simply reset the freelist, which
doesn't work for tests that require actual objects in the space.
The version for new space also doesn't work because it assumes
everything after top is free space.

Fill the space with FixedArray by iterating over the freelist and
creating an object in place of each freelist entry.
This method actually fills the space, so that we can also use it to
force page promotion.

Bug: v8:12612
Change-Id: Ie0d73e846bbf688ea52030be29e0587b2f37ed4e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3823135Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82437}