tools: testing: selftests: prefer TEST_PROGS for conntrack_dump_flush

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.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>