Add basic model tests

tests/data/attachment.json

+{
+   "uuid": "21a70f8a-8ea3-47a9-b775-f128199b699c",
+   "size": 73601,
+   "mimetype": "image/png",
+   "creation_date": "2022-09-05T10:31:49.267845Z",
+   "urls": {
+      "source": "https://am.pirateradio.social/media/attachments/61/fa/50/logo.png",
+      "original": "https://s3.j19.planetexpress.cc/georg-funkwhale/attachments/4c/1c/d7/logo.png?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=georg-funkwhale%2F20220925%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20220925T150226Z&X-Amz-Expires=3600&X-Amz-SignedHeaders=host&X-Amz-Signature=6eef479e6394d7a6fc380720d1b509d86dcd80140754ab4efb173b90d7e9c0b5",
+      "medium_square_crop": "https://s3.j19.planetexpress.cc/georg-funkwhale/__sized__/attachments/4c/1c/d7/logo-crop-c0-5__0-5-200x200.png?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=georg-funkwhale%2F20220925%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20220925T150226Z&X-Amz-Expires=3600&X-Amz-SignedHeaders=host&X-Amz-Signature=e5e130966df8da983bdfa85ff13914ce034486ca5f336b51fa06ae90db68c624",
+      "large_square_crop": "https://s3.j19.planetexpress.cc/georg-funkwhale/__sized__/attachments/4c/1c/d7/logo-crop-c0-5__0-5-600x600.png?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=georg-funkwhale%2F20220925%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20220925T150226Z&X-Amz-Expires=3600&X-Amz-SignedHeaders=host&X-Amz-Signature=bd7598bcb5a339715c294fc11cf18082fca993340a10ccd876f21632c43ea9f9"
+   }
+}
\ No newline at end of file