Run tests on a weekly schedule

Also allow manual workflow dispatches.

Pull requests and pushes still create each job twice for a push to an
open pull request within the same repo, but fixing that is non-trivial
and I frankly don't care enough.

Author: FichteFoll

.github/workflows/test.yml

@@ -1,6 +1,11 @@
 name: Tests
 
-on: [push, pull_request]
+on:
+  push:
+  pull_request:
+  schedule:
+    - cron: "* 0 * * SAT"
+  workflow_dispatch:
 
 jobs:
   no_defpkg: