Skip unnecessary workflows to save resources 🌳

[GollyTicker]

Almost always the test workflow is run twice on the same code - once during the PR review - and a second time after the merge into master.

This is unnecessary and puts extra costs to GitHub and other computing resources.

I would like to add and use Skip Duplicate Actions in places, where the same code is tested multiple times.

We are interested in the following non-exhaustive changes:

• canceling the test workflow for all non-recent runs
• skip tests for the same run after merge into main
  • Fortunately, the skip only happens for the same code state - and it is still run for manual invocations. This seems appropriate here.