Regularly run tests

This makes spotting OS regressions/behaviour changes much more timely,
and avoids breaking random PRs.

Author: martinpitt

.github/workflows/tests.yml

@@ -1,5 +1,9 @@
 name: unit-tests
-on: [push, pull_request]
+on:
+  push:
+  pull_request:
+  schedule:
+  - cron: 0 4 * * MON,FRI
 jobs:
   OS:
     runs-on: ubuntu-latest