Route the filename arg of foreign-load/save ops through a /tmp file instead of a random path from the input (load) or /dev/null (save).

Unlocks coverage of code that memory sources are unable to reach: Mostly libvips/iofuncs/source.c (e.g. mmap
fast paths).

Temp files are written lazily and g_unlinked per iteration, so ops without a filename arg aren't slowed down.

ToDo:

• Improve the seed corpus for filename operations further
• Check for operations that aren't cleaned up correctly (VipsForeignSaveDz might be a candidate) and block them
• Local test fuzzing run