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

Here is the test result after conversion.

 # ./icmp_redirect.sh

 ###########################################################################
 Legacy routing
 ###########################################################################

 TEST: IPv4: redirect exception                                      [ OK ]

 ...

 TEST: IPv4: mtu exception plus redirect                             [ OK ]
 TEST: IPv6: mtu exception plus redirect                             [ OK ]

 Tests passed:  40
 Tests failed:   0
 Tests xfailed:   0
Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>