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

Here is the test result after conversion.

 ]# ./vrf_strict_mode_test.sh

 ################################################################################
 TEST SECTION: VRF strict_mode test on init network namespace
 ################################################################################

     TEST: init: net.vrf.strict_mode is available                        [ OK ]

     TEST: init: strict_mode=0 by default, 0 vrfs                        [ OK ]

 ...

     TEST: init: check strict_mode=1                                     [ OK ]

     TEST: testns-HvoZkB: check strict_mode=0                            [ OK ]

 Tests passed:  37
 Tests failed:   0

Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>