selftests: net: include forwarding lib

selftests: net: include forwarding lib

The altnames test uses the forwarding/lib.sh and that dependency
currently causes failures when running the test after install:

  make -C tools/testing/selftests/ TARGETS=net install
  ./tools/testing/selftests/kselftest_install/run_kselftest.sh \
      -t net:altnames.sh
  # ...
  # ./altnames.sh: line 8: ./forwarding/lib.sh: No such file or directory
  # RTNETLINK answers: Operation not permitted
  # ./altnames.sh: line 73: tests_run: command not found
  # ./altnames.sh: line 65: pre_cleanup: command not found

Address the issue leveraging the TEST_INCLUDES infrastructure
provided by commit 2a0683be5b4c ("selftests: Introduce Makefile variable
to list shared bash scripts")

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/f7b1e9d468224cbc136d304362315499fe39848f.1707298927.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>