Create integration test for Fleet managed manual rollback #11534
Description
After merging elastic/fleet-server#5975 and #11143 implement an integration test where a manual rollback is triggered for a Fleet managed agent both within and without grace period.
As a rough guide, TestStandaloneUpgradeManualRollback can be used with what is being done to test fleet-managed automatic rollbacks.
This cannot be done as part of #11143 since fleet managed upgrades require valid signatures for the downloaded packages and there's no way to disable the verification when upgrading from Fleet (there's no equivalent to standalone upgrade --skip-verify switch and skipping verification in DEV builds has been removed a long time ago)