make slab reassign tests more reliable
make slab reassign tests more reliable

on 32bit hardware with different pointer/slab class sizes, the tests would
fail. made a few adjustments to ensure reassign rescues happen and make items
not be near the default slab class borders.

This makes the tests pass, but needs further improvements for reliability..
ie: "fill until evicts", count slab pages for reassignment/etc.

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