selftests: net: Fix cross-tree inclusion of scripts

When exporting and running a subset of selftests via kselftest, files from
parts of the source tree which were not exported are not available. A few
tests are trying to source such files. Address the problem by using
symlinks.

The problem can be reproduced by running:
make -C tools/testing/selftests gen_tar TARGETS="drivers/net/bonding"
[... extract archive ...]
./run_kselftest.sh

or:
make kselftest KBUILD_OUTPUT=/tmp/kselftests TARGETS="drivers/net/bonding"

Fixes: bbb774d9 ("net: Add tests for bonding and team address list management")
Fixes: eccd0a80 ("selftests: net: dsa: add a stress test for unlocked FDB operations")
Link: https://lore.kernel.org/netdev/40f04ded-0c86-8669-24b1-9a313ca21076@redhat.com/


Reported-by: Jonathan Toppins <jtoppins@redhat.com>
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Reviewed-by: Jonathan Toppins <jtoppins@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>