- test_unmap_simple: was assuming that zero buffer was already cleared, ...

...which is not guaranteed and resulted in spurious failures

- writesame10_unmap_until_end, writesame16_unmap, writesame16_unmap_until_end were doing an CU_ASSERT *PER-BYTE* in the verification phase,
which was very CPU-intensive. This change uses memcmp on a whole block which finishes much quicker.