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

Here is the test result after conversion.

]# ./test_vxlan_under_vrf.sh
Checking HV connectivity                                           [ OK ]
Check VM connectivity through VXLAN (underlay in the default VRF)  [ OK ]
Check VM connectivity through VXLAN (underlay in a VRF)            [ OK ]

Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>