So here's the new test class that handles the test from
groups set. The class is inherited from zdtm_test one as
what it does is -- starts the pseudo-init in ns/uns flavors
and asks one to spawn() the sub-tests from the list.

All groups tests can only be run inside ns flavor, so if
the host flavor is asked, just the pseudo-init is spawned.
This is because using ns flavor is the easiest way to spawn
all the sub tests under this init (however, h flavor can be
supported by marking the pseudo-init as sub-reaper).

On stop this pseudo-init is signalled to stop, it in turn
stops all the sub-tests and then exits. When the pid
namespace destruction is complete, the sub-tests .out-s are
checked.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>