testsuite: Generate nlmsg blob at runtime

testsuite: Generate nlmsg blob at runtime

Since netlink messages are in host byte order, shipping a pre-generated
nlmsg blob won't suffice on systems with different endianness. Therefore
generate the blob at runtime, so it's content fits the hosts endianness.

Note that the generated message will contain only a single interface
featuring two VFs instead of the full list before. Yet this is
sufficient, as it triggers the crash with iproute versions prior to
commit 8c29ae7cc2494 ("ip link: Fix crash on older kernels when show VF
dev").

Signed-off-by: Phil Sutter <phil@nwl.cc>