feat: enhance ethtool-test.py workflow

- Add color-coded output, detailed per-interface summaries, and DHCP
renewal when default routes are missing. Implement graceful cleanup
to restore original interface states after execution

- Include optional IPv6 ping and fallback link detection via sysfs
when ethtool is absent

- This improves script portability, test coverage, and readability

Signed-off-by: Manjunatha D <[email protected]>
Change-Id: I49c83fa3be940cb3d4faeca2476b8864d005be51
(cherry picked from commit 68b4e37eecb3ed0709f1ea7d9c4bb61b2aa539ec)

Author: manjunath3divakar