selftests/net: add lib.sh

commit 25ae948b upstream.

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>
[PHLin: add lib.sh to TEST_FILES directly as we already have upstream
        commit 06efafd8 landed in 6.6.y]
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>