Add ability to run expensive tests every 24 hours

#2473105: Update hook_help texts that link to modules that can be uninstalled is a test that the docs folks are writing to ensure hook_help() both is and stays in good shape. However, it does so by installing/uninstalling every module individually and checking its help page to make sure it both exists and doesn't throw any errors.

Since this test is expensive (along with #2258241: Remove obsolete Drupal\system\Tests\Module\InstallUninstallTest) I understand it causes headaches for DrupalCI patch review testing times. So one thought to mitigate that was to be able to somehow mark "expensive" tests and only run them once per day, like we've suggested for SQLite and PostgreSQL.