Refactor VerifyReachability test case to add coverage for DF Bit, packet size

[vitthalmagadum]

We need to refactor the existing VerifyReachability test case to enhance coverage including additional scenarios. Currently the test case does not cover:

1. DF (Don't Fragment) Bit: We need to test packets with the DF bit set to ensure proper handling of fragmentation.
2. Packet sizes: The test case should cover a range of packet size to verify behavior under different conditions.

Tasks:

• Update the test case to include optional field of DF bit(Update input model for df-bit) in command.
• Include optional field of packet size variation(Update input model for packet size) in command.

Example of ping commands: ping $dest_ip source Loopback0 size 9000 df-bit repeat 10 ping $LO0P_IP source Loopback0 size 9100 df-bit repeat 10