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: I...