selftests: cgroup/memcontrol: add basic test for swap controls

The new test verifies that memory.swap.max and memory.swap.current behave
as expected for simple allocation scenarios

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