BUG: fix uint alignment asserts in lowlevel loops #12655
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Backport of #12626.
Further correction to the debug assert statements in lowlevel_stride_loops.c.src to account for uint alignment, see #12618.
This also updates the unit test so it always fails if the alignment is incorrectly calculated, instead of sporadically failing depending on what malloc gives. That's done by making _aligned_zeros align to the requested alignment yet not twice the alignment.
The particular case that was failing was for 16-byte longdouble, which is 8-byte "uint aligned" but 16-byte "true aligned". (The copy-code copies 16-byte types with two uint64 assignments). So an 8-byte-aligned ptr would go into the uint aligned copy code, but would trip the 16-byte assert statement.