Set default testEnvironment to 'node'

[VolkerSchiewe]

First of all: create library, very nice to use!

We are using mongoose in our tests and where always wondering why it was printing the warning Mongoose: looks like you're trying to test a Mongoose app with Jest's default jsdom test environment. while this library sets the testEnvironment to node. Now I found that the testEnvironment in the config is not actually used, because it is overwritten by the code in test.js.
Hence this pull request 😊

[VolkerSchiewe]

I just noticed that it is possible to get around this problem, because the implementation is based on 'create react app': adding the flag --env=node. In any case I think changing the default behaviour makes sense because this is what most people will expect from this library.

[jayair]

Ah good catch. Thanks for the PR!