chore: remove string-to-arraybuffer test dependency

[tinovyatkin]

This PR removes external test dependency string-to-arraybuffer in favor of Node.js built-in util.TextEncoder available since Node 8.30

[jimmywarting]

I approve.

fyi, here is something i picked up long ago:

var encode = TextEncoder.prototype.encode.bind(new TextEncoder())
encode('text').buffer

nicer to have then having to construct a new encoder all the time and reuse the same one. But the way you have done it is okey also.

[NotMoni]

lgtm