feat(anta): Refactor VerifyReachability test case to add coverage for DF Bit, packet size

[vitthalmagadum]

Description

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

Fixes #756

Checklist:

• [X] My code follows the style guidelines of this project
• [X] I have performed a self-review of my code
• [X] I have run pre-commit for code linting and typing (pre-commit run)
• [ ] I have made corresponding changes to the documentation
• [X] I have added tests that prove my fix is effective or that my feature works
• [X] New and existing unit tests pass locally with my changes (tox -e testenv)