selftests/net: convert traceroute.sh to run it in unique namespace

Here is the test result after conversion.

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

Tests passed:   2
Tests failed:   0

Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>