selftests: net: improved IOAM tests

As previously discussed with David Ahern, here is a refactored and improved
version of the IOAM self-test. It is now more complete and more robust. Now,
all tests are divided into three categories: OUTPUT (evaluates the IOAM
processing by the sender), INPUT (evaluates the IOAM processing by the receiver)
and GLOBAL (evaluates wider use cases that do not fall into the other two
categories). Both OUTPUT and INPUT tests only use a two-node topology (alpha and
beta), while GLOBAL tests use the entire three-node topology (alpha, beta,
gamma). Each test is documented inside its own handler in the (bash) script.

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Signed-off-by: David S. Miller <davem@davemloft.net>