The watchdog previously didn't terminate execution, it just prevented
the execution of additional tasks.
This CL fixes that by making {TaskRunner::Terminate} actually terminate
execution in the isolate.

It also adds a regression test for this.

R=szuend@chromium.org

Bug: chromium:1154412, chromium:1142437
Change-Id: Ic6638e8a5c37e8840a85651b4d4bea2ee0f71c43
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2622212Reviewed-by: Simon Zünd <szuend@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72031}