selftests/net: add lib.sh

Add a lib.sh for net selftests. This file can be used to define commonly
used variables and functions. Some commonly used functions can be moved
from forwarding/lib.sh to this lib file. e.g. busywait().

Add function setup_ns() for user to create unique namespaces with given
prefix name.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>