Add an always_sparkplug testing variant, and fix a couple of issues it
found.

Bug: v8:11420
Change-Id: I7d87a41e3413f40271a0140118531f075d633b23
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2773047Reviewed-by: Patrick Thier <pthier@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73529}