test: skip speed capability test if not reported.

on some interfaces ethtool is not able to report the advertised speed
capabilities. Therefore, such tests should be skipped.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
test: add interface offload modify tests.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
test: print the right interface name in test title

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>