cppgc: Add tests for in-construction during ctor

Adds explicit tests that check that an object is marked as in
construction while running the constructor.

Bug: chromium:1056170
Change-Id: I7f7340832e1bc31cec98784c261ed86deb402e72
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2811238


Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73869}