Here's my preferred approach, with breadcrumbs kept in erb views:

Make this view helper.

    def breadcrumb(&)
      render(layout: 'common/breadcrumb', &)
    end

Add this partial 'common/_breadcrumb.html.erb' (do whatever html you want):

    <li class="breadcrumb-item">
      <%= yield %>
    </li>

Add this to your layout:

    <% if content_for?(:breadcrumbs) %>
      <ol class="breadcrumbs">
        <%= yield :breadcrumbs %>
      </ol>
    <% else %>

Then this is how you use it in your views:

    <% content_for :breadcrumbs do %>
      <%= breadcrumb { link_to 'Foo', foo_url } %>
      <%= breadcrumb { link_to 'Bar', bar_url } %>
      <%= breadcrumb { 'you are here' } %>
    <% end %>

For minitest tests I add this helper:

    module AssertBreadcrumbs
      Crumb = Struct.new(:text, :href)

      # Note: the block must have 1 argument per breadcrumb. It asserts their count.
      def assert_breadcrumbs(&blk)
        assert_select '.breadcrumb-item', blk.parameters.size do |items|
          structs = items.map { |item|
            if (link = item.css('a')[0])
              Crumb.new(link.text, link['href'])
            else
              Crumb.new(item.text.strip)
            end
          }

          yield(*structs)
        end
      end
    end

Which you can use in tests like this:

    assert_breadcrumbs do |item1, item2, item3|
      assert_equal 'Foo', item1.text
      assert_equal foo_url, item1.href

      assert_equal 'Bar', item2.text
      assert_equal bar_url, item2.href

      assert_equal 'you are here', item3.text
      assert_nil item3.href
    end