Improve testing on schedule

[jku]

We run some tests on schedule even now but when those tests fail, nothing happens.

We could do this:

• Make current test workflows callable
• Add new scheduled-tests workflow that
  • runs on schedule
  • calls all test workflows that should be run on schedule (ci, conformance, cross-os, cross-version, staging-tests maybe)
  • files an issue if anything fails

[jku]

see also #1601