Add utils to easily test graphql server

[tobiasdiez]

Based on @nuxt/test-uilts, similar to the ones introduced in JabRef/JabRefOnline#2376 but better be based on https://github.com/jasonkuhrt/graphql-request (needs to use the next version of "Graffle" for the custom fetch function see graffle-js/graffle#1012)