selftests/net: convert vrf-xfrm-tests.sh to run it in unique namespace

Here is the test result after conversion.

]# ./vrf-xfrm-tests.sh

No qdisc on VRF device
TEST: IPv4 no xfrm policy                                           [ OK ]
TEST: IPv6 no xfrm policy                                           [ OK ]
TEST: IPv4 xfrm policy based on address                             [ OK ]
TEST: IPv6 xfrm policy based on address                             [ OK ]
TEST: IPv6 xfrm policy with VRF in selector                         [ OK ]
TEST: IPv4 xfrm policy with xfrm device                             [ OK ]
TEST: IPv6 xfrm policy with xfrm device                             [ OK ]

netem qdisc on VRF device
TEST: IPv4 no xfrm policy                                           [ OK ]
TEST: IPv6 no xfrm policy                                           [ OK ]
TEST: IPv4 xfrm policy based on address                             [ OK ]
TEST: IPv6 xfrm policy based on address                             [ OK ]
TEST: IPv6 xfrm policy with VRF in selector                         [ OK ]
TEST: IPv4 xfrm policy with xfrm device                             [ OK ]
TEST: IPv6 xfrm policy with xfrm device                             [ OK ]

Tests passed:  14
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>