zdtm.sh: execute all test in NEW_PIDNS in addition (v2)

Now the most part of test cases may be executed in pidns.
By default all test cases are executed in current pidns, then
they are executed in new pidns again.

v2: remove the hard coded path to  test_init

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