Add tests for the example apps

[n1ru4l]

I do not want that the examples break without anyone noticing.

What to do:

• Extract the express instance into a separated file that exports it
• Test the exported express instance with supertest (https://github.com/visionmedia/supertest) and jest for some basic endpoint functionality.

How to do:

• Setup project (https://github.com/n1ru4l/graphql-schema-generator-rest#contribute)
• Add jest and supertest as a dependency to the examples
• Write some simple tests that prove that all features which are showcased in the example app are working

Related paths:

• https://github.com/n1ru4l/graphql-schema-generator-rest/tree/master/examples/express-complex
• https://github.com/n1ru4l/graphql-schema-generator-rest/tree/master/examples/express