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

Here is the test result after conversion.

]# ./test_vxlan_nolocalbypass.sh
TEST: localbypass enabled                                           [ OK ]
TEST: Packet received by local VXLAN device - localbypass           [ OK ]
TEST: localbypass disabled                                          [ OK ]
TEST: Packet not received by local VXLAN device - nolocalbypass     [ OK ]
TEST: localbypass enabled                                           [ OK ]
TEST: Packet received by local VXLAN device - localbypass           [ OK ]

Tests passed:   6
Tests failed:   0

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>