Add test coverage for 404 on throbber-active.gif and friends

Problem/Motivation

Follow-up for 🐛 [regression] 404 for misc/throbber-active.gif / misc/loading-small.giff Fixed . Quoting @catch:

For me this fits into 🌱 [policy, no patch] Better scoping for bug fix test coverage RTBC - i.e. I think we should open an issue to add test coverage. The throbber is afaik untestable in that it's by definition only visible during loading so js tests can't reliably find it. Maybe tablesort would be more straightforwardly testable? But then how do we assert CSS images are reference correctly in a test, especially without hardcoding the new path? Feels like something for CSS regression testing which we don't have.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet