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

There is no h1 h2 actually. Remove it. Here is the test result after
conversion.

]# ./test_bridge_backup_port.sh

Backup port
-----------
TEST: Forwarding out of swp1                                        [ OK ]
TEST: No forwarding out of vx0                                      [ OK ]
TEST: swp1 carrier off                                              [ OK ]
TEST: No forwarding out of swp1                                     [ OK ]
...
Backup nexthop ID - ping
------------------------
TEST: Ping with backup nexthop ID                                   [ OK ]
TEST: Ping after disabling backup nexthop ID                        [ OK ]

Backup nexthop ID - torture test
--------------------------------
TEST: Torture test                                                  [ OK ]

Tests passed:  83
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>