Trigger tests when opening a pull request

Author: asafgabai

.github/workflows/tests.yml

@@ -6,6 +6,7 @@ on:
     tags-ignore:
       - '**'
   workflow_dispatch:
+  pull_request_target:
 # Ensures that only the latest commit is running for each PR at a time.
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}-${{ github.ref }}