Fix test_alignment_of_different_layouts on architectures with 128-bit vectors

This test previously failed on ARM Neon (and probably also on SSE) because the test checks for 256-bit alignment, but only requests alignment=True, which is the maximum required by the hardware (i.e. to align=32 on AVX, but only align=16 on SSE/Neon).

assigned to @holzer

merged

mentioned in commit 37518a47