FillCurrentPage assumed that everything after top is empty, which
doesn't work with MinorMC and sweeping. Revise FillCurrentPage based
SimulateFullSpace for MinorMC.

I similar implementation is provided both in unittests and cctest.
Migrating affected cctest to unittests is left a future work.

Bug: v8:12612
Change-Id: Ie29be2fc7aaee25e1fd5f66b1c0959c2a45f007f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3885888Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83158}