Test: Add integration tests for chat

[benbucksch]

• Start with git commit 2b71d74 on branch stanza.
• Make the backend start up automatically (right now, you have to start it manually)
• Find a way to store the test account credentials (email address and password), without checking it into the git repo. Could be e.g. a github secret variable which contains JSON, and is then reflected as environment variable, which is then read by the test, and parsed as JSON.
• Set up GitHub Action to run these tests. For now, start manually (on: workflow_dispatch, see example). Later, might run it 4 times per day.