This assertion was borked, as it accepted obviously "same" values like
the same object. This fixes the predicate by switching both assertSame
and assertNotSame to use {Object.is} underneath. It also adds a new
respective regression test (gotta test the tester).

R=ahaas@chromium.org
TEST=message/mjsunit/fail/assert_not_same

Change-Id: I6ba20c4b8b96a736ab924715b1cad78f2f43a120
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1687541Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62512}