selftests: forwarding: Allow lib.sh sourcing from other directories

The devlink related scripts are mlxsw-specific. As a result, they'll
reside in a different directory - but would still need the common logic
implemented in lib.sh.
So as a preliminary step, allow lib.sh to be sourced from other
directories as well.

Signed-off-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>