workflows: run test on push

Run the test workflow on push rather than on pull_request.  This ensures
that it runs in other forks of the repository too, keeping contributors
honest.  And the workflow will still only run once.

.github/workflows/test.yml

@@ -1,6 +1,6 @@
 name: test
 
-on: pull_request
+on: push
 
 jobs:
   test: