contextify: use offset/length from Uint8Array

Do not blindly take data from underlying `ArrayBuffer`, use
`ByteOffset`/`ByteLength` of `Uint8Array` itself.

Additionally, fix tests that weren't actually properly running because
of V8's internal code cache. The code should be different, otherwise the
cached data won't be used at all.

Fix: #4939
PR-URL: https://github.com/nodejs/node/pull/4947


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>