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: NJiri Pirko <jiri@mellanox.com>
Signed-off-by: NIdo Schimmel <idosch@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

tools/testing/selftests/drivers/net/mlxsw/spectrum/resource_scale.sh

@@ -2,7 +2,6 @@
 # SPDX-License-Identifier: GPL-2.0
 
 NUM_NETIFS=6
-source ../../../../net/forwarding/lib.sh
 source ../../../../net/forwarding/tc_common.sh
 source devlink_lib_spectrum.sh