CI: Also run tests on stacked pull requests

Author: amotl

.github/workflows/tests.yml

@@ -1,10 +1,9 @@
 name: Tests
 
 on:
+  pull_request: ~
   push:
     branches: [ main ]
-  pull_request:
-    branches: [ main ]
 
   # Also, run each Monday At 07:00.
   schedule: