While adding tests I found that metaball export has been broken since
Blender 3.4. It would export each metaball geometry twice.
This looks to have been a side effect of a change to `object_dupli.cc`
which no longer sets the `no_draw` flag for metaballs[1]. With the flag
unset we would end up visiting this particular object twice.
Use a direct check for Metaballs now and add test coverage for the
scenario in general.
[1] eaa87101cd
Pull Request: https://projects.blender.org/blender/blender/pulls/131984
afe0ebeb74