add tests for asset loaders

## What problem does this solve or what need does it fill?

Some asset loaders (for example, gltf) are getting complex and are hard to tests as calling the loader function typically needs an asset server

## What solution would you like?

Figure out how to add tests for asset loaders

## Additional context

This was raised in #3512

Start by reviewing the asset-loader code and the discussion in issue #3512, then trace how loaders such as gltf currently obtain an asset server. Done means asset loaders can be exercised in tests without requiring an asset server, with tests covering the relevant loader behavior.

Written by the indexing model from the issue text.