Update spec test for relative url

spec/presenters/packages/composer/packages_presenter_spec.rb

@@ -67,10 +67,14 @@ RSpec.describe ::Packages::Composer::PackagesPresenter do
       {
         'packages' => [],
         'provider-includes' => { 'p/%hash%.json' => { 'sha256' => /^\h+$/ } },
-        'providers-url' => "/api/v4/group/#{group.id}/-/packages/composer/%package%$%hash%.json"
+        'providers-url' => "prefix/api/v4/group/#{group.id}/-/packages/composer/%package%$%hash%.json"
       }
     end
 
+    before do
+      stub_config(gitlab: { relative_url_root: 'prefix' })
+    end
+
     it 'returns the provider json' do
       expect(subject).to match(expected_json)
     end