test: dm: eth: Add ip6_addr_in_subnet test

Add a test if two address are in the same subnet. Use in sandbox

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>