Run tests on schedule (daily)

To make sure that all dependencies still work

.github/workflows/tests.yml

@@ -7,6 +7,8 @@ on:
   push:
     branches: master
   pull_request:
+  schedule: # Run daily at 08:00 CEST (06:00 UST)
+    - cron: '0 6 * * *'
 
 ###############
 # Set the Job #