tests: iptables-test: extend coverage for ip6tables

tests: iptables-test: extend coverage for ip6tables

Update iptables-test.py to run libxt_*.t both for iptables and
ip6tables. For libxt_*.t tests, append the command name to status output
line. This update requires changes in the existing tests.

* Rename libxt_*.t into libipt_*.t and add libip6_*.t variant.

- TEE
- TPROXY
- connlimit
- conntrack
- iprange
- ipvs
- policy
- recent

* Rename the following libxt_*.t to libipt_*.t since they are IPv4
  specific:

- standard
- osf

* Remove IPv4 specific test in libxt_mark.t

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Phil Sutter <phil@nwl.cc>