ci: run on all branches

(not sure why, it stopped running on PRs...)

Author: olivierdalang

.github/workflows/tests.yml

@@ -1,8 +1,5 @@
 on:
-  pull_request:
   push:
-    branches:
-      - master
   schedule:
     - cron:  '0 6 * * 1'  # weekly check