Added cross-env so tests can be runned on Windows

[ArjanSchouten]

First of all, thanks for creating this nice package manager :+1: !

Checklist

• [x] tests and code linting passes (the tests will pass on unix. On Windows the tests will still fail although it is now possible to actually run them with npm test).
• [x] the commit message follows commit guidelines

  Affected core subsystem(s)

Testing and development

Description of change

The scripts in the package.json will set the NODE_ENV variable. This isn't working on Windows since Windows users have to use set NODE_ENV=test. The package cross-env takes care of setting environment variables on multiple platforms.

[mgcrea]

Looks good to me.

[alexanderGugel]

Wonderful! Thanks!