Add a basic e2e-test for `skia_gold_client`. (#51078)

Basically, I take blank images of red/blue/green squares, like this:

![solid_red_square](https://github.com/flutter/engine/assets/168174/b1de63e4-8c3c-43ff-9bcf-b067a7e9d920)

... and use Image Magick to annotate it with the current git hash:

```sh
$ drt ./testing/skia_gold_client/tool/generate.dart

Writing annotation "8069cb4ca1" on images in testing/skia_gold_client/tool/source_images and saving them in testing/skia_gold_client/tool/e2e_fixtures.
Writing to testing/skia_gold_client/tool/e2e_fixtures/solid_red_square.png
Writing to testing/skia_gold_client/tool/e2e_fixtures/solid_green_square.png
Writing to testing/skia_gold_client/tool/e2e_fixtures/solid_blue_square.png
Done: wrote 3 image.
```

![solid_red_square](https://github.com/flutter/engine/assets/168174/0ece31a8-9e64-44a2-ad06-78c04b02650c)

... then, I upload the digests very similar to how we do it in `scenario_app`.

---

The idea here is to have a way for me to know if Skia gold is working the way I/we expect, independent of a more complicated test suite with various race-y or flake-y conditions. We could also augment it with tests of "dimensions" properties.