tools: testing: selftests: prefer TEST_PROGS for conntrack_dump_flush

Currently conntrack_dump_flush test program always runs when passing
TEST_PROGS argument:

% make -C tools/testing/selftests TARGETS=net/netfilter \
 TEST_PROGS=conntrack_ipip_mtu.sh run_tests
make: Entering [..]
TAP version 13
1..2 [..]
  selftests: net/netfilter: conntrack_dump_flush [..]

Move away from TEST_CUSTOM_PROGS to avoid this.  After this,
above command will only run the program specified in TEST_PROGS.

Link: https://lore.kernel.org/netdev/20240423191609.70c14c42@kernel.org/Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240424095824.5555-1-fw@strlen.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/netfilter/Makefile

@@ -28,10 +28,9 @@ TEST_PROGS += nft_zones_many.sh
 TEST_PROGS += rpath.sh
 TEST_PROGS += xt_string.sh
 
-TEST_CUSTOM_PROGS += conntrack_dump_flush
+TEST_GEN_PROGS = conntrack_dump_flush
 
 TEST_GEN_FILES = audit_logread
-TEST_GEN_FILES += conntrack_dump_flush
 TEST_GEN_FILES += connect_close nf_queue
 TEST_GEN_FILES += sctp_collision