selftests: forwarding: tc_actions: Use ncat instead of nc

selftests: forwarding: tc_actions: Use ncat instead of nc

The test relies on 'nc' being the netcat version from the nmap project.
While this seems to be the case on Fedora, it is not the case on Ubuntu,
resulting in failures such as [1].

Fix by explicitly using the 'ncat' utility from the nmap project and the
skip the test in case it is not installed.

[1]
 # timeout set to 0
 # selftests: net/forwarding: tc_actions.sh
 # TEST: gact drop and ok (skip_hw)                                    [ OK ]
 # TEST: mirred egress flower redirect (skip_hw)                       [ OK ]
 # TEST: mirred egress flower mirror (skip_hw)                         [ OK ]
 # TEST: mirred egress matchall mirror (skip_hw)                       [ OK ]
 # TEST: mirred_egress_to_ingress (skip_hw)                            [ OK ]
 # nc: invalid option -- '-'
 # usage: nc [-46CDdFhklNnrStUuvZz] [-I length] [-i interval] [-M ttl]
 #         [-m minttl] [-O length] [-P proxy_username] [-p source_port]
 #         [-q seconds] [-s sourceaddr] [-T keyword] [-V rtable] [-W recvlimit]
 #         [-w timeout] [-X proxy_protocol] [-x proxy_address[:port]]
 #         [destination] [port]
 # nc: invalid option -- '-'
 # usage: nc [-46CDdFhklNnrStUuvZz] [-I length] [-i interval] [-M ttl]
 #         [-m minttl] [-O length] [-P proxy_username] [-p source_port]
 #         [-q seconds] [-s sourceaddr] [-T keyword] [-V rtable] [-W recvlimit]
 #         [-w timeout] [-X proxy_protocol] [-x proxy_address[:port]]
 #         [destination] [port]
 # TEST: mirred_egress_to_ingress_tcp (skip_hw)                        [FAIL]
 #       server output check failed
 # INFO: Could not test offloaded functionality
 not ok 80 selftests: net/forwarding: tc_actions.sh # exit=1

Fixes: ca22da2fbd69 ("act_mirred: use the backlog for nested calls to mirred ingress")
Reported-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Closes: https://lore.kernel.org/netdev/adc5e40d-d040-a65e-eb26-edf47dac5b02@alu.unizg.hr/
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Tested-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20230808141503.4060661-12-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>