selftests: cgroup/memcontrol: add basic test for socket accounting

The test verifies that when there is active TCP connection, the
memory.stat.sock and memory.current values are close.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>