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

Here is the test result after conversion.

]# ./drop_monitor_tests.sh

Software drops test
    TEST: Capturing active software drops                               [ OK ]
    TEST: Capturing inactive software drops                             [ OK ]

Hardware drops test
    TEST: Capturing active hardware drops                               [ OK ]
    TEST: Capturing inactive hardware drops                             [ OK ]

Tests passed:   4
Tests failed:   0

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