selftests: net: replace sleeps in fcnal-test with waits

fcnal-test.sh already includes lib.sh, use relevant helpers
instead of sleeping. Replace sleep after starting nettest
as a server with wait_local_port_listen.

Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250909223837.863217-1-kuba@kernel.org


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