selftests: traceroute: Return correct value on failure

selftests: traceroute: Return correct value on failure

The test always returns success even if some tests were modified to
fail. Fix by converting the test to use the appropriate library
functions instead of using its own functions.

Before:

 # ./traceroute.sh
 TEST: IPV6 traceroute                                               [FAIL]
 TEST: IPV4 traceroute                                               [ OK ]

 Tests passed:   1
 Tests failed:   1
 $ echo $?
 0

After:

 # ./traceroute.sh
 TEST: IPv6 traceroute                                               [FAIL]
         traceroute6 did not return 2000:102::2
 TEST: IPv4 traceroute                                               [ OK ]
 $ echo $?
 1

Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20250908073238.119240-5-idosch@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>