CTS Memory tests fix invalid allocation sizes

CTS Memory tests fix invalid allocation sizes

allocateRandom() routine sometime requests enormous sizes
because of incorrect random size calculation logic.

Within the following formula:
m_heap.size / MAX_MEMORY_USAGE_DIV) - m_usage - 1ull
If m_heap.size / MAX_MEMORY_USAGE_DIV) - m_usage is "0"
the result is negative, but since the number is unsigned
max takes the max of 1 and the (unsigned long long)-1.

The change, makes the comparison signed and also introduces
explicit test checks on the allocation sizes.