selftests: netfilter: conntrack_tcp_unreplied.sh: wait for initial connection attempt

Netdev CI reports occasional failures with this test
("ERROR: ns2-dX6bUE did not pick up tcp connection from peer").

Add explicit busywait call until the initial connection attempt shows
up in conntrack rather than a one-shot 'must exist' check.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240506114320.12178-1-fw@strlen.de


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