Improve how locales and prompt file name interact with each other

The safe test downloads prompt files from modellab. It supports multiple locales in a given file, but doesn't do well with different files for different locales.

Workarounds exist, but they're hacky.

Options:

• Improve file naming conventions to support locales. Proposal: if a file includes one locale, include that locale in the filename. English prompt files don't include en_us and that makes the code messy.
• Ensure that we don't try to make test items in locale X using a file that does not include locale X (e.g. in the smoke test).
• Handle file not found errors better, especially in unit or smoke tests.
• If any unit tests rely on downloading an asset from modellab, replace that with a local file.

References:

Workaround for test items

Workaround for filenames with and without locales