Add tests using npm pack

[ctavan]

Add a test using npm pack, see

• https://medium.com/javascript-in-plain-english/is-promise-post-mortem-cab807f18dcc
• https://github.com/then/is-promise/pull/25/files#diff-1d37e48f9ceff6d8030570cd36286a61R126

This was brought up in #423 (comment)