Update specs for header component

In GOV.UK Component for Ruby on Rails 6.0.0 the class for the homepage
link in the header was changed [[1]], to match the same change in GOV.UK
Frontend 6.0.0 [[2]].

This commit updates our specs to use the new class, as they were relying on
it to be able to pick out and test the homepage link.

[1]: x-govuk/govuk-components@255e51e
[2]: alphagov/govuk-frontend@9f4728d

Author: lfdebrux

spec/components/form_header_component/view_spec.rb

@@ -15,7 +15,7 @@
   it "links to the GOV.UK homepage" do
     render_inline(described_class.new(current_context:, mode:))
 
-    expect(page.find("a.govuk-header__link--homepage")[:href]).to eq "https://www.gov.uk/"
+    expect(page.find("a.govuk-header__homepage-link")[:href]).to eq "https://www.gov.uk/"
   end
 
   it "links to the form start page" do
@@ -55,7 +55,7 @@
 
       render_inline(described_class.new(current_context:, mode:))
 
-      expect(page.find(".govuk-header__link--homepage")[:href]).to eq "http://forms-admin/"
+      expect(page.find(".govuk-header__homepage-link")[:href]).to eq "http://forms-admin/"
     end
   end
 
@@ -75,7 +75,7 @@
 
       render_inline(described_class.new(current_context:, mode:))
 
-      expect(page.find(".govuk-header__link--homepage")[:href]).to eq "http://forms-admin/"
+      expect(page.find(".govuk-header__homepage-link")[:href]).to eq "http://forms-admin/"
     end
   end