selftests/net/forwarding: add missing tests to Makefile

When generating the selftests to another folder, the fixed tests are
missing as they are not in Makefile, e.g.

  make -C tools/testing/selftests/ install \
  	TARGETS="net/forwarding" INSTALL_PATH=/tmp/kselftests

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>