Ilya noticed that the test fails on recent kernels.
He also wrote:

"Marcelo pointed out the following change that went into 6.5 kernel:
https://lore.kernel.org/netdev/20230626064749.75525-7-pablo@netfilter.org/

It looks like this test is not going to work anymore as new connections
are now allowed to evict closing ones on collisions.

We need to re-think the test, i.e. we have to keep the first connection
open while we're trying to make the second one, so it is not evicted."