Separate stress testing from the poison block testing

[rhempel]

Currently the stress testing mechanism is built into the poison block testing. This needs an awkward data structure to manage calling the correct version of the allocation functions. Even worse, the stress test results are slightly different for 8 byte blocks and higher block sizes due to the extra memory that must be allocated to handle the poison areas.
Ideally, we need to separate the stress testing from poison testing, and also update the poison data area size to correctly allocate extra memory depending on the block size to that the number of blocks allocated during poison testing remains the same regardless of the block size.