Use config.assets.prefix for asset paths in a spec

[fenec]

Summary

Sometimes, assets path differ from /assets, so it's better not to hardcode it in tests. Using Rails.application.config.assets.prefix instead

Checklist

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.00%. Comparing base (8e003e7) to head (8d8afda). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #370 +/- ## ======================================= Coverage 95.99% 96.00% ======================================= Files 207 207 Lines 4874 4876 +2 ======================================= + Hits 4679 4681 +2 Misses 195 195 ```

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[kennyadsl]

Thanks @fenec!