selftests/net: allow GRO coalesce test on veth

This change extends the existing GRO coalesce test to
allow running on top of a veth pair, so that no H/W dep
is required to run them.

By default gro.sh will use the veth backend, and will try
to use exiting H/W in loopback mode if a specific device
name is provided with the '-i' command line option.

No functional change is intended for the loopback-based
tests, just move all the relevant initialization/cleanup
code into the related script.

Introduces a new initialization helper script for the
veth backend, and plugs the correct helper script according
to the provided command line.

Additionally, enable veth-based tests by default.

v1 -> v2:
  - drop unused code in setup_veth_ns() - Willem

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>