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

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

Here is the test result after conversion.

 # ./unicast_extensions.sh
 /usr/bin/which: no nettest in (/root/.local/bin:/root/bin:/usr/share/Modules/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin)
 ###########################################################################
 Unicast address extensions tests (behavior of reserved IPv4 addresses)
 ###########################################################################
 TEST: assign and ping within 240/4 (1 of 2) (is allowed)            [ OK ]
 TEST: assign and ping within 240/4 (2 of 2) (is allowed)            [ OK ]
 TEST: assign and ping within 0/8 (1 of 2) (is allowed)              [ OK ]

 ...

 TEST: assign and ping class D address (is forbidden)                [ OK ]
 TEST: routing using class D (is forbidden)                          [ OK ]
 TEST: routing using 127/8 (is forbidden)                            [ OK ]

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