Update tests for design to support default branch

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/331906

app/services/design_management/save_designs_service.rb

@@ -119,7 +119,7 @@ module DesignManagement
     # Returns the latest blobs for the designs as a Hash of `{ Design => Blob }`
     def existing_blobs
       @existing_blobs ||= begin
-        items = designs.map { |d| ['HEAD', d.full_path] }
+        items = designs.map { |d| [target_branch, d.full_path] }
 
         repository.blobs_at(items).each_with_object({}) do |blob, h|
           design = designs.find { |d| d.full_path == blob.path }

spec/controllers/projects/design_management/designs/raw_images_controller_spec.rb

@@ -109,7 +109,7 @@ RSpec.describe Projects::DesignManagement::Designs::RawImagesController do
 
         context 'when sha is nil' do
           let(:sha) { nil }
-          let(:expected_ref) { 'master' }
+          let(:expected_ref) { project.design_repository.root_ref }
 
           it_behaves_like 'a successful request for sha'
         end

spec/services/design_management/copy_design_collection/copy_service_spec.rb

@@ -191,8 +191,8 @@ RSpec.describe DesignManagement::CopyDesignCollection::CopyService, :clean_gitla
             expect(commits_on_master(limit: 99)).to include(*target_issue.design_versions.ordered.pluck(:sha))
           end
 
-          it 'creates a master branch if none previously existed' do
-            expect { subject }.to change { target_repository.branch_names }.from([]).to(['master'])
+          it 'creates a default branch if none previously existed' do
+            expect { subject }.to change { target_repository.branch_names }.from([]).to([project.design_repository.root_ref])
           end
 
           it 'does not create default branch when one exists' do
@@ -255,7 +255,7 @@ RSpec.describe DesignManagement::CopyDesignCollection::CopyService, :clean_gitla
           end
 
           def commits_on_master(limit: 10)
-            target_repository.commits('master', limit: limit).map(&:id)
+            target_repository.commits(target_repository.root_ref, limit: limit).map(&:id)
           end
         end
       end

spec/services/design_management/save_designs_service_spec.rb

@@ -177,6 +177,18 @@ RSpec.describe DesignManagement::SaveDesignsService do
         end
       end
 
+      context 'when HEAD branch is different from master' do
+        before do
+          stub_feature_flags(main_branch_over_master: true)
+        end
+
+        it 'does not raise an exception during update' do
+          run_service
+
+          expect { run_service }.not_to raise_error
+        end
+      end
+
       context 'when a design is being updated' do
         before do
           run_service
@@ -343,7 +355,7 @@ RSpec.describe DesignManagement::SaveDesignsService do
           path = File.join(build(:design, issue: issue, filename: filename).full_path)
           design_repository.create_if_not_exists
           design_repository.create_file(user, path, 'something fake',
-                                        branch_name: 'master',
+                                        branch_name: project.default_branch_or_main,
                                         message: 'Somehow created without being tracked in db')
         end