zdtm: do not run cow0x tests in userns

Opening /proc/<pid>/pagemap in userns is only available since linux
v4.3 or later, after commit 1c90308e7a77 (pagemap: hide physical
addresses from non-privileged users). So those tests fail on older
kernels:
https://ci.openvz.org/job/CRIU/job/CRIU-virtuozzo/job/criu-dev/1402

travis-ci: success for zdtm: do not run cow0x tests in userns
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

test/zdtm/static/cow00.desc

 # /proc/pid/pagemap doesn't show phys addr for unprivileged users
-{'flavor': 'ns h uns', 'flags': 'suid'}
+{'flavor': 'ns h', 'flags': 'suid'}

test/zdtm/static/cow01.desc

-{'flavor': 'h ns uns', 'flags': 'suid'}
+{'flavor': 'h ns', 'flags': 'suid'}