tc-testing: add test for ct DNAT tuple collision

When this test fails, /proc/net/nf_conntrack gets only 1 entry:
ipv4     2 tcp      6 119 SYN_SENT src=10.0.0.10 dst=10.0.0.10 sport=5000 dport=10 [UNREPLIED] src=20.0.0.1 dst=10.0.0.10 sport=10 dport=5000 mark=0 secctx=system_u:object_r:unlabeled_t:s0 zone=0 use=2

When it works, it gets 2 entries:
ipv4     2 tcp      6 119 SYN_SENT src=10.0.0.10 dst=10.0.0.20 sport=5000 dport=10 [UNREPLIED] src=20.0.0.1 dst=10.0.0.10 sport=10 dport=58203 mark=0 secctx=system_u:object_r:unlabeled_t:s0 zone=0 use=2
ipv4     2 tcp      6 119 SYN_SENT src=10.0.0.10 dst=10.0.0.10 sport=5000 dport=10 [UNREPLIED] src=20.0.0.1 dst=10.0.0.10 sport=10 dport=5000 mark=0 secctx=system_u:object_r:unlabeled_t:s0 zone=0 use=2

The missing entry is because the 2nd packet hits a tuple collusion and the
conntrack entry doesn't get allocated.

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>