test: dm: eth: Add string_to_ip6 test

Add a test to check convertation from char* to struct in6_addr.
Use in sandbox

Series-changes: 3
- Fixed tests to use length param in string_to_ip6()

Series-changes: 5
- Add test under #ifdef

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>