test: dm: eth: Add csum_ipv6_magic test

Test checksum computation. csum_ipv6_magic() uses in upper layer
protocols as TCP/UDP/ICMPv6/etc to calculate payload checksum.

Series-changes: 3
- Fixed style problems

Signed-off-by: Viacheslav Mitrofanov <v.v.mitrofanov@yadro.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>