selftests: netfilter: query conntrack state to check for port clash resolution

Jakub reported this self test flaking occasionally (fails, but passes on
re-run) on debug kernels.

This is because the test checks for elapsed time to determine if both
connections were established in parallel.

Rework this to no longer depend on timing.
Use busywait helper to check that both sockets have moved to established
state and then query the conntrack engine for the two entries.

Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/netfilter-devel/20250926163318.40d1a502@kernel.org/


Fixes: 117e149e ("selftests: netfilter: test nat source port clash resolution interaction with tcp early demux")
Signed-off-by: Florian Westphal <fw@strlen.de>