selftests/net: convert l2tp.sh to run it in unique namespace

Here is the test result after conversion.

]# ./l2tp.sh
TEST: IPv4 basic L2TP tunnel                                        [ OK ]
TEST: IPv4 route through L2TP tunnel                                [ OK ]
TEST: IPv6 basic L2TP tunnel                                        [ OK ]
TEST: IPv6 route through L2TP tunnel                                [ OK ]
TEST: IPv4 basic L2TP tunnel - with IPsec                           [ OK ]
TEST: IPv4 route through L2TP tunnel - with IPsec                   [ OK ]
TEST: IPv6 basic L2TP tunnel - with IPsec                           [ OK ]
TEST: IPv6 route through L2TP tunnel - with IPsec                   [ OK ]
TEST: IPv4 basic L2TP tunnel                                        [ OK ]
TEST: IPv4 route through L2TP tunnel                                [ OK ]
TEST: IPv6 basic L2TP tunnel - with IPsec                           [ OK ]
TEST: IPv6 route through L2TP tunnel - with IPsec                   [ OK ]
TEST: IPv4 basic L2TP tunnel - after IPsec teardown                 [ OK ]
TEST: IPv4 route through L2TP tunnel - after IPsec teardown         [ OK ]
TEST: IPv6 basic L2TP tunnel - after IPsec teardown                 [ OK ]
TEST: IPv6 route through L2TP tunnel - after IPsec teardown         [ OK ]

Tests passed:  16
Tests failed:   0

Acked-by: David Ahern <dsahern@kernel.org>
Reviewed-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>