Set entries return an array with the value as first and second entry.
As such these are considered key value pairs to align with maps
entries iterator.
So far the return value was identical to the values iterator and that
is misleading.

This also adds tests to verify the results and improves the coverage
a tiny bit by testing different iterators.

Refs: https://github.com/nodejs/node/issues/24629

R=yangguo@chromium.org

Change-Id: I669a724bb4afaf5a713e468b1f51691d22c25253
Reviewed-on: https://chromium-review.googlesource.com/c/1350790
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59311}