tests: Fix test allocator to provide fundamental alignment
tests: Fix test allocator to provide fundamental alignment

Previously test allocator only guaranteed alignment enough for a pointer.

On some platforms (e.g. SPARC) double has to be aligned to 8 bytes but pointers
can have a size of 4 bytes. This commit increases allocation header to fix that.

In practical terms the allocation header is now always 8 bytes.

zeux/pugixml,zeux/pugixml,zeux/pugixml,zeux/pugixml