tests: Cancel in-progress tests on PR updates

Before this commit GitHub Action runs that were triggered by a PR were
not canceled when updates were made to the same PR. This lead to even
more clogging of our pipelines and not enough runners being available.

This changes the behavior in a way that whenever a PR gets updated all
still-in-progress runs get canceled and new runs get spawned.

.github/workflows/test.yml

@@ -2,6 +2,10 @@ name: Tests
 
 on: [pull_request, push]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   test_suite:
     name: "Unittest"