zdtm: add new test case for testing shared memory

* map a big memory region
* fork
* unmap the first and tenth pages
* remap one page in another place
* touch a few pages
* check content of touched pages

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