[GCU] Enabling AddRack and adding RemoveRack tests (sonic-net#2143)

#### What I did
Fixes sonic-net#2051

RemoveRack/AddRack is confirmed to be working correctly by Renuka. Check sonic-net/sonic-mgmt#5254

#### How I did it
- Enabled ADD_RACK test
- Created REMOVE_RACK test using opposite data to ADD_RACK, verified the generated steps manually.

#### How to verify it
UnitTest and the KVM test by Renuka.

#### Previous command output (if the output of a command-line utility has changed)

#### New command output (if the output of a command-line utility has changed)

Author: ghooo