selftests: netfilter: Pass family parameter "-f" to conntrack tool

selftests: netfilter: Pass family parameter "-f" to conntrack tool

Fix nft_conntrack_helper.sh false fail report:

1) Conntrack tool need "-f ipv6" parameter to show out ipv6 traffic items.

2) Sleep 1 second after background nc send packet, to make sure check
is after this statement executed.

False report:
FAIL: ns1-lkjUemYw did not show attached helper ip set via ruleset
PASS: ns1-lkjUemYw connection on port 2121 has ftp helper attached
...

After fix:
PASS: ns1-2hUniwU2 connection on port 2121 has ftp helper attached
PASS: ns2-2hUniwU2 connection on port 2121 has ftp helper attached
...

Fixes: 619ae8e0697a6 ("selftests: netfilter: add test case for conntrack helper assignment")
Signed-off-by: Chen Yi <yiche@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>