tests: sleep: Increase tick margin for Xilinx QEMU

The Xilinx QEMU, used to emulate the Xilinx ZynqMP platform, is
particularly unstable in terms of timing.

This commit increases the tick margin for the Xilinx ZynqMP platform
from 1 to 5 in order to allow the sleep test to pass with a reasonable
repeatability.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>