selftests: mlxsw: avoid double sourcing of lib.sh

Don't source lib.sh 2 times and make the script work with ifnames
passed on the command line.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>