commit 25ae948b ("selftests/net: add lib.sh") added net/lib.sh to
contain code shared by tests under net/ and net/forwarding/. However, this
caused issues with selftests from directories other than net/forwarding/,
in particular those under drivers/net/. Those issues were avoided in a
simple way by duplicating some content in commit 2114e833 ("selftests:
forwarding: Avoid failures to source net/lib.sh").

In order to remove the duplicated content, restore the inclusion of
net/lib.sh from net/forwarding/lib.sh but with the following changes:

* net/lib.sh is imported through the net_forwarding_dir path
The original expression "source ../lib.sh" would look for lib.sh in the
directory above the script file's one, which did not work for tests under
drivers/net/.

* net/lib.sh is added to TEST_INCLUDES
Since net/forwarding/lib.sh now sources net/lib.sh, both of those files
must be exported along with tests which source net/forwarding/lib.sh.
Suggested-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>