Enable pull requests to be run from v*test branches (#41474)

Since we switch from direct push of cherry-picking to open PRs
against v*test branch, we should enable PRs to run for the target
branch.

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ on:  # yamllint disable-line rule:truthy
   push:
     branches: ['v[0-9]+-[0-9]+-test']
   pull_request:
-    branches: ['main']
+    branches: ['main', 'v[0-9]+-[0-9]+-test']
   workflow_dispatch:
 permissions:
   # All other permissions are set to none