selftests: net: List helper scripts in TEST_FILES Makefile variable

Some scripts are not tests themselves; they contain utility functions used
by other tests. According to Documentation/dev-tools/kselftest.rst, such
files should be listed in TEST_FILES. Move those utility scripts to
TEST_FILES.

Fixes: 1751eb42ddb5 ("selftests: net: use TEST_PROGS_EXTENDED")
Fixes: 25ae948b4478 ("selftests/net: add lib.sh")
Fixes: b99ac1841147 ("kselftests/net: add missed setup_loopback.sh/setup_veth.sh to Makefile")
Fixes: f5173fe3e13b ("selftests: net: included needed helper in the install targets")
Suggested-by: Petr Machata <[e-mail]>
Signed-off-by: Benjamin Poirier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/Makefile

···
 TEST_PROGS += ip_local_port_range.sh
 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_PROGS_EXTENDED := toeplitz_client.sh toeplitz.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
···
 TEST_PROGS += vlan_hw_filter.sh
 
 TEST_FILES := settings
+TEST_FILES += in_netns.sh lib.sh net_helper.sh setup_loopback.sh setup_veth.sh
 
 include ../lib.mk