Harden specs for repositories storage weights.

What does this MR do and why?

This MR refactors two specs related to storage weights to prevent bugs from sneaking in when changing the underlying implementation of ApplicationSettingsHelper#storage_weights.

Screenshots or screen recordings

n/a

How to set up and validate locally

$ tests=$(tff -f tests.yml $(git diff --name-only origin/master))
$ echo $tests
spec/helpers/application_settings_helper_spec.rb spec/views/admin/application_settings/_repository_storage.html.haml_spec.rb

$ bin/rspec $tests
.........................................

Finished in 9.48 seconds (files took 8 seconds to load)
41 examples, 0 failures

Randomized with seed 43658

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.