Failover using preferred order - What happens when we force failover ?

The preferred order will not change but rather the next active device will. In your example, bigip-test-1 will become active and bigip-test-2 will be the next active device. Also note, that you have auto failback enabled. The traffic-group-2 will automatically failover back to bigip-test-2 after 60 seconds of being available.

So you mean IF I run the failover command on bigip-test-2 ....bigip-test-1 will become active but after 60sec it will again failover to bigip-test-2 as auto failback is configured ?

That is correct.

8. If you configured the Failover Methods setting with a value of Failover using Preferred Device List and then Load Aware, then configure the following settings. Otherwise, skip this step.

1. Select or clear the check box for the Auto Failback option.
2. If Auto Failback is selected, then in the Auto Failback Timeout field, type the number of seconds that you want the system to wait before failing back to the specified device. The range in seconds is from 0 to 300. The default is 60. A value of 40 to 60 seconds allows for state mirroring information to be re-mirrored for traffic groups.

https://techdocs.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/bigip-system-device-service-clustering-administration-13-1-0/6.html