custom webpack configuration + qunit testing

[MichaelS86]

Make sure to check the existing issues in this repository

Please, tell us what's the problem?

[ ] A typo
[ ] Wrong documentation
[ ] Improvement of existing article
[ x ] Missing documentation
[ ] New article needed

Please, tell us more details

On the webpage https://docs.nativescript.org/tooling/custom-webpack-configuration there is a detailed description on how to use a separate webpack-config-file for custom issues.
(At least) When using QUnit, there is also a karma.conf.js-file that loads the webpack-config, which might also be adapted.
A description on issues regarding karma.conf.js when using a custom webpack configuration, however, is missing.

In my recent exemplary case, I made use of dotenv in the webpack-configuration. The expected environment variables, however, were not available during testing, since the testing framework still made use of the standard webpack-config-file. It took me some time to find the reason for that.