Only trigger tests on PR (#15)

Co-authored-by: openhands <openhands@all-hands.dev>

Author: xingyaoww

.github/workflows/tests.yml

@@ -1,6 +1,6 @@
 name: Run tests
 on:
-  push:
+  pull_request:
     branches: ["**"]
   pull_request:
     branches: ["**"]