selftests: net: More graceful finding of `ip'.

The ip tool might be provided by another package (such as
Busybox), not necessarily implementing the -Version switch.
Trying an actual usage (`ip link show') might be a better
test that would work with all implementations of `ip'.

Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>