zdtm: Add CLOCK_BOOTTIME test into posix_timers

To test CLOCK_BOOTTIME feature recently implemented in OpenVZ kernel.
Vanilla kernel and CRIU passes it.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>