ogt_vox : add formal test cases

[jpaver]

• to prevent regressions with further dev, we'd like coverage on: -- files produced by various magicavoxel versions. eg. early versions that don't produce nSHP/nTRN/nGRP chunks. -- groups vs flattening -- animation sampling
• we may also want to try some fuzzer testing.

[mgerhardy]

would you mind me adding a few tests - and adding googletest and https://github.com/google/fuzztest?

[mgerhardy]

for benchmarking this model might work well - as it's quite big: https://github.com/Phyronnaz/VoxelAssets/tree/master/Oasis - and CC0 licensed