[C++] Test for alignment issues

### Describe the enhancement requested

#14488 / #33336 are about an issue where we fail to handle unaligned buffers properly. We should see if we can test for these kinds of issues more systematically. For example: maybe we could run tests under an allocator that intentionally misaligns allocations.

### Component(s)

C++

Start by reading issues #14488 and #33336 to understand the unaligned-buffer failures and the expected coverage. Then identify the existing C++ allocation and test infrastructure; done means tests systematically detect incorrect handling of intentionally misaligned buffers, with the scope and allocator approach agreed.

Written by the indexing model from the issue text.