selftests/net/forwarding: define libs as TEST_PROGS_EXTENDED

The lib files should not be defined as TEST_PROGS, or we will run them
in run_kselftest.sh.

Also remove ethtool_lib.sh exec permission.

Fixes: 81573b18 ("selftests/net/forwarding: add Makefile to install tests")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>