zdtm: add a new test case for checking vma-s with the MAP_GROWSDOWN flags

Here are two case:
* check that a vma grows after restoring
* check that a vma can be restored if its guard page was overlapped by
  another vma.

Currently criu isn't able to dump/restore this test case. Who wants to
fix this issue?

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>