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>

selftests: net: Add drop monitor test

Test that drop monitor correctly captures both software and hardware
originated packet drops.

# ./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

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>