Issue #1821530 by LinL, alexpott: Move cron key from configuration to state system.

...egator/lib/Drupal/aggregator/Tests/AggregatorCronTest.php

.../modules/simpletest/lib/Drupal/simpletest/WebTestBase.php

core/modules/system/config/system.cron.yml

...les/system/lib/Drupal/system/Tests/System/CronRunTest.php

...rupal/system/Tests/Upgrade/StateSystemUpgradePathTest.php

...lib/Drupal/system/Tests/Upgrade/SystemUpgradePathTest.php

core/modules/system/system.install

core/modules/system/system.module

...s/system/tests/upgrade/drupal-7.state.system.database.php