selftests/net: fix reinitialization of TEST_PROGS in net self tests.

Assinging will drop all previous tests.

Fixes: b690842d ("selftests/net: test l2 tunnel TOS/TTL inheriting")
Signed-off-by: Adel Abouchaev <adel.abushaev@gmail.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20220824184351.3759862-1-adel.abushaev@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/Makefile

@@ -42,7 +42,7 @@ TEST_PROGS += arp_ndisc_evict_nocarrier.sh
 TEST_PROGS += ndisc_unsolicited_na_test.sh
 TEST_PROGS += arp_ndisc_untracked_subnets.sh
 TEST_PROGS += stress_reuseport_listen.sh
-TEST_PROGS := l2_tos_ttl_inherit.sh
+TEST_PROGS += l2_tos_ttl_inherit.sh
 TEST_PROGS += bind_bhash.sh
 TEST_PROGS_EXTENDED := in_netns.sh setup_loopback.sh setup_veth.sh
 TEST_PROGS_EXTENDED += toeplitz_client.sh toeplitz.sh