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

Here is the test result after conversion.

 ]# ./vrf_route_leaking.sh

 ###########################################################################
 IPv4 (sym route): VRF ICMP ttl error route lookup ping
 ###########################################################################

 TEST: Basic IPv4 connectivity                                       [ OK ]
 TEST: Ping received ICMP ttl exceeded                               [ OK ]

 ...

 TEST: Basic IPv6 connectivity                                       [ OK ]
 TEST: Traceroute6 reports a hop on r1                               [ OK ]

 Tests passed:  18
 Tests failed:   0
Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>