[tsp-client] Only run test action for PRs targeting main (Azure#29843)

- Only branch guaranteed to have specs used for testing

.github/workflows/tsp-client-test.yaml

@@ -5,6 +5,8 @@ on:
     branches:
       - main
   pull_request:
+    branches:
+      - main
     paths:
       - package-lock.json
       - package.json