Since there is no `Value::IsInteger` method in the API, we in the
Node.js project are going to rely on what looks like an implementation
detail of the Integer class. It is currently possible to to call
`Integer::Value` on any Number and the value is cast to an integer.
This commit adds tests for this behavior.

Change-Id: I4de09e7c6e0beac7909e5477f7bfe2ed4c9415b9
Reviewed-on: https://chromium-review.googlesource.com/1200983
Commit-Queue: Michaël Zasso <mic.besace@gmail.com>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55629}