Automating import and export tests

[glenrobson]

We could test the following:

• Import all manifests without python errors
• Import manifest, export and check default values
• Multilingual metadata makes it through import and appears in export
• Add test fixtures with multi language labels

[vincentmarchetti]

The testing of importing manifests is addressed in PR #37

See successful test result https://github.com/IIIF-Commons/iiif_3d_blender_plugin/actions/runs/14999413740/job/42142041599?pr=37 ; in particular the "Run Tests" item