Replace generic ".breadcrumbs" selector in specs with find_by_testid

What does this MR do and why?

This MR removes any finders looking for .breadcrumbs in our feature specs and replaces it with a find_by_testid/within_testid.

Why?

Short answer: We should use testids in our specs, not such generic CSS classnames.

Longer answer: This is also a preparation step for #358113 where our current Haml rendering of breadcrumbs will be replaced by Vue.js and I want to completely drop that .breadcrumbs CSS class from the new implementation. But currently 27 spec files complain about this, so better get that refactored ahead of time 🤓

How to set up and validate locally

• Only existing feature specs got updated. These should pass, obviously.
• If you want to see where this old .breadcrumbs class is renderend, see app/views/layouts/nav/breadcrumbs/_breadcrumbs.html.haml#L16. On the same element is also the breadcrumb-links testid that is now used instead.