[wasm-gc] Refactor GC tests

Add some abstractions to make it easier to define more tests.

Bug: v8:7748
Change-Id: Ia5605aa10963228eb4bfba37e2b412fc5af860d8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2224212
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68147}