support/runttime-tests: fix openssh test

When it was applied, commit 243d500f (support/testing: add openssh
runtime test) was amended to not provide a NIC to the emulated machine,
as the test did not require access to the outer world: it only uses the
lo interface. Also, there was a discrepancy between the NIC name in the
Buildroot configuration, and the drivers available in our default kernel
image, making the boot hang for a while whaiting for a NIC that would
never come.

However, that tweak was tested locally with a qmeu version more recent
than the one available in our buidroot/base Docker image. As a
consequence, that test fails to run in gitlab-ci.

Revert to using the old way of specifying no network: it works on
gitlab-ci, and qemu versions in standard distros still support it.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>