We can test HA failover, by shutdown or rebooting of ESXi hosts,
ideally to test Failover, better to pull the power cord out of the host, that will be actual failure of the host. If you are using nested environment then Power Off (not guest shutdown) your ESXi VM to do this activity. Controlled reboot/shutdown might not give you desired effect all the time.
Is it possible to test failback, After failure ESXi host has been recovered and running fine
Will the VMs migrate back to the original hosts(automatically) or we need to perform vMotion for migrating to the original host
There's no such thing as Failback in vSphere HA, so if your host returns to active duty, it will be empty (without any VMs) to begin with. so Manual vMotion migration is going to be needed. But let's say your cluster was also configured with Fully Auto DRS, then you might notice that some VMs got auto migrated to your recovered Host, don't expect same set of VMs to return though.