Previously, unhandled promise rejections weren't reset between REPRL
executions, leading to incorrect exit statuses being reported. This CL
fixes the issue and adds further tests to verify the correct behaviour.

Change-Id: Ied47d9359b0fbc05ebb211667687a0a4041ef767
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2431205Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Samuel Groß <saelo@google.com>
Cr-Commit-Position: refs/heads/master@{#70227}