Rewrite requesttoken.spec.js with Jest

[Jest](https://jestjs.io/

) is a test runner that focuses on simplicity.
It instruments babel to transform modules and test them.

Using Jest simplifies the existing configuration and allows dropping a
bunch of workarounds, as well as following the shared Babel
configuration for new code.

Signed-off-by: François Freitag <mail@franek.fr>