Fix off-thread space merging unit tests, by

  a) Doing a GC on each test teardown (avoiding leaking GC state between
     tests), and
  b) Removing the AllocationStep when merging, which could cause a
     deadlock if incremental marking triggered a free-list refill.

Bug: v8:10142
Change-Id: Id2670115f96ab3b13613ea357bd44a639d0151e1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2142260
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67077}