Configure "Functional Tests" properly

[fabasoad]

Is your feature request related to a problem? Please describe

The problem is that currently there are no tests for this GitHub Action configured.

Describe the solution you'd like

Since this GitHub Action works only with the changed files compared to base, the setup might be slightly more difficult than usual. Currently, I am thinking about the following:

1. CI workflow 1 is triggered on pull_request/push (as usual) and:
   1. Adds new image
   2. Creates new branch and pushes changes into it
   3. Branch should be with the certain pattern, e.g. ci/*
2. CI workflow 2 is triggered on pushes to ci/* branches and:
   1. Runs GitHub Action on top of new branch
   2. Deletes branch in the very last step

Ideally, on step 1.1 should be also renaming, modification and copying to validate all scenarios. But at least added files is already good enough.

Describe alternatives you've considered

None.