Run functional tests against production build

[michaelkleinhenz]

Currently, the functional tests are run against an intermediate build. We're using the WebPack dev/debug function for that. Only after they succeed, the final build is done and the output artifact created. The reason is that we don't need a test-only http-server setup as WebPack already includes that.

I found out that errors caused by problems in master may appear ONLY when doing a production build, NOT when doing a debug/dev build. The result is, that the problems are completely invisible in a dev environment and for the functional tests.

We need to check if we can run the functional tests against the production build instead of the intermediate build.