selftests: net: included needed helper in the install targets

The blamed commit below introduce a dependency in some net self-tests
towards a newly introduce helper script.

Such script is currently not included into the TEST_PROGS_EXTENDED list
and thus is not installed, causing failure for the relevant tests when
executed from the install dir.

Fix the issue updating the install targets.

Fixes: 3bdd9fd2 ("selftests/net: synchronize udpgro tests' tx and rx connection")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/076e8758e21ff2061cc9f81640e7858df775f0a9.1706131762.git.pabeni@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/Makefile

@@ -55,6 +55,7 @@ TEST_PROGS += rps_default_mask.sh
 TEST_PROGS += big_tcp.sh
 TEST_PROGS_EXTENDED := in_netns.sh setup_loopback.sh setup_veth.sh
 TEST_PROGS_EXTENDED += toeplitz_client.sh toeplitz.sh lib.sh
+TEST_PROGS_EXTENDED += net_helper.sh
 TEST_GEN_FILES =  socket nettest
 TEST_GEN_FILES += psock_fanout psock_tpacket msg_zerocopy reuseport_addr_any
 TEST_GEN_FILES += tcp_mmap tcp_inq psock_snd txring_overwrite