Restrict PR testing to main branch

Signed-off-by: nmburgan <13688219+nmburgan@users.noreply.github.com>

Author: nmburgan

.github/workflows/tests.yaml

@@ -2,7 +2,7 @@
 name: RSpec tests
 
 on:
-  pull_request: {}
+  pull_request: { branches: ['main'] }
   push:
     branches:
       - main