Add progress bar to loader-tests

[Kamino0]

Fixes #143 Added progress bar to loader-tests.

Also changed & to & in example.opml to satisfy parser. Recently there was a fix to the сontentType definition and on my computer queryselectorAll started returning an empty list. I think the problem in more strict parsing of XML in comparison with HTML, since previously the example.opml file was defined as text/html and now as application/xml. I suppose in future resolving issue #195 fix this problem.

Checklist

• [x] Don’t rush. Check all changes in PR again.
• [x] Run pnpm test.
• [x] Think about changing documentation.
  • If you added a script to scripts/, add a comment with a description.
  • If you added a new folder, add its description to the project’s README.md.
  • If you added config, describe how we use this tool in the config’s comment.
  • If you added something to the project’s architecture, describe it in the project’s README.md.
  • Try to focus on “why?”, not “how?”.
• [x] If you added a new dependency, check our requirements.
• [x] Think about testing
  • If you added a feature, add unit tests.
  • If you added a new state to the UI, add visual tests.
  • If you fixed the bug, think about preventing bug regression in the future.
• If you changed web client:
  • [ ] Think about moving code to core/. What code will also be useful on other platforms?
  • [ ] Run pnpm size and check the difference in the JS bundle size. Is it relevant to the changes? Change the limit in web/.size-limit.json if necessary.
  • [ ] The UI was checked in Chrome and Firefox (and Safari or Epiphany if you have them).
  • [ ] Think about keyboard UX. Is it easy to use the new feature with only one hand on a keyboard? Is it easy to understand what keys to press?
  • [ ] Think about HTML semantics.
  • [ ] Think about accessibility. Check a11y recommendations. Think about how screen reader users will use the tool. Is it easy to use on a screen with bad contrast?
  • [ ] Think about translations.
  • [ ] Think about right-to-left languages. What parts of the screen should be mirrored for Arabic or Hebrew languages?
• If you changed web/main/ files:
  • Do you need to backport styles changes to web/public/404.html or web/public/500.html?
  • Think about app loading styles inlined in index.html.
• If you changed core/:
  • [ ] Think about making types more precise. Can you better explain data relations by type?
  • [ ] Think about conflict resolution. We don’t need some very smart changing merging; just 2 changes of the same item on different clients should not break the database. What if the user changes an item on one machine and removes it on another?
  • [ ] Think about log and storage migration.
• If you changed English translations:
  • [ ] Change translation ID if you change the meaning of the text.

[ai]

Can you show screenshot of progress bar?

[Kamino0]

Can you show screenshot of progress bar?

Added gif to main comment

[ai]

Thanks! Great job.