selftests: dsa: Replace test symlinks by wrapper script

The dsa tests which are symlinks of tests from net/forwarding/ (like
tc_actions.sh) become regular files after export (because `rsync
--copy-unsafe-links` is used) and expect to source lib.sh
(net/forwarding/lib.sh) from the same directory.

In the last patch of this series, net/forwarding/lib.sh will source lib.sh
from its parent directory (ie. net/lib.sh). This would not work for dsa
tests because net/lib.sh is not present under drivers/net/.

Since the tests in net/forwarding/ are not meant to be copied and run from
another directory, as a preparation for that last patch, replace the test
symlinks by a wrapper script which runs the original tests under
net/forwarding/. Following from that, the links to shared library scripts
in dsa/ are no longer used so remove them and add all the original files
needed from parent directories to TEST_INCLUDES.

Suggested-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Tested-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>