selftests: forwarding: Set default IPv6 traceroute utility

The test uses the 'TROUTE6' environment variable to encode the name of
the IPv6 traceroute utility. By default (without a configuration file),
this variable is not set, resulting in failures:

 # ./ip6_forward_instats_vrf.sh
 TEST: ping6                                                         [ OK ]
 TEST: Ip6InTooBigErrors                                             [ OK ]
 TEST: Ip6InHdrErrors                                                [FAIL]
 TEST: Ip6InAddrErrors                                               [ OK ]
 TEST: Ip6InDiscards                                                 [ OK ]

Fix by setting a default utility name and skip the test if the utility
is not present.

Fixes: 0857d6f8 ("ipv6: When forwarding count rx stats on the orig netdev")
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/20230808141503.4060661-6-idosch@nvidia.com


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