selftests: netfilter: fix conntrack stress test failures on debug kernels

selftests: netfilter: fix conntrack stress test failures on debug kernels

Jakub reports test failures on debug kernel:
FAIL: proc inconsistency after uniq filter for ...

This is because entries are expiring while validation is happening.

Increase the timeout of ctnetlink injected entries and the
icmp (ping) timeout to 1h to avoid this.

To reduce run-time, add less entries via ctnetlink when KSFT_MACHINE_SLOW
is set.

also log of a failed run had:
 PASS: dump in netns had same entry count (-C 0, -L 0, -p 0, /proc 0)

... i.e. all entries already expired: add a check and set failure if
this happens.

While at it, include a diff when there were duplicate entries and add
netns name to error messages (it tells if icmp or ctnetlink failed).

Fixes: d33f889fd80c ("selftests: netfilter: add conntrack stress test")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/netdev/20250506061125.1a244d12@kernel.org/
Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://patch.msgid.link/20250507075000.5819-1-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>