Properly test the abstract equality - both JSEqual and JSNotEqual - for
the case of symbols. Also add tests for the corner cases of the
JSObjectIsArray operator, which is used to implement Array.isArray()
builtin.

Bug: v8:8015
Change-Id: Ib008e85553d04527a5992a904ec77774761f872e
Reviewed-on: https://chromium-review.googlesource.com/1238237Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56121}