The tests need to properly hold on to the original fast-mode map,
otherwise the GC might clear that, and so the NormalizedMapCache
lookup would fail due to that.

Bug: chromium:963411, v8:9114, v8:9183, v8:9267
Change-Id: Ic41ed363959a5c182c74097767dc14c366076e17
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1627333Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61772}