update ci-testing.yml with on: cron (#572)

.github/workflows/ci-testing.yml

 name: CI CPU testing
 
-# see: https://help.github.com/en/actions/reference/events-that-trigger-workflows
-on: [push, pull_request]
+on:  # https://help.github.com/en/actions/reference/events-that-trigger-workflows
+  push:
+  pull_request:
+  schedule:
+    - cron: "0 0 * * *"
 
 jobs:
   cpu-tests: