Monorepo testing section to include details about jest `moduleNameMapper` configuration needed for e2e jest config file in each application

[sebastiangug]

The title pretty much includes all the details, I'll try to contribute it myself & make a pull request with this.

Essentially, when doing e2e testing in a monorepo you need to configure the moduleNameMapper property in the config file targeted by the test command, otherwise @app imports will not be resolved properly and initalisation will fail.

Took me a couple of hours to debug this although that might be just me sucking at realizing which jest configuration is being picked up & how module resolution is being handled for @app imports.