Update test.yml to be reusable

This will allow us to re-use the same workflow file in all of our repos instead of having to constantly update it in each one when we want to tweak the matrix.

Author: adammathys

.github/workflows/test.yml

@@ -7,6 +7,7 @@ on:
   pull_request:
   schedule:
     - cron: "0 0 * * 4" # every Thursday
+  workflow_call:
 
 concurrency:
   group: test-${{ github.ref_name }}