Avoid design flaky specs

Some specs that create a design with a `with_file` trait would fail
intermittently with a Gitaly error `A file with this name already
exists`. This signalled that a collision in the design.full_path in
the spec. The full_path is made from the id of the issue and its name.

The failing tests would fail when a `create(:design, :with_file)` was

- created in a `before` block
- its issue was created in a `set`
-  and it was setting a specific filename for the design.

In this situation, if a capybara spec failed once, it would retry again,
and would try to create a design with the same name and save it against
the same issue from the last time it ran. This would lead to a full_path
conflict.

The fix is a combination of:
- using the default (dynamically sequenced) filenames for designs that
  are generated by FactoryBot where possible
- if we do set a design that has a `with_file` trait (meaning it will
  create an underlying commit in Gitaly for the design version) then the
  issue must be created at the same time as the design is within the
  test

See https://gitlab.com/gitlab-org/gitlab/issues/32620#note_222564584

ee/spec/features/projects/issues/design_management/user_paginates_designs_spec.rb

@@ -9,8 +9,7 @@ describe 'User paginates issue designs', :js do
   before do
     enable_design_management
 
-    create(:design, :with_file, issue: issue, filename: 'world.png')
-    create(:design, :with_file, issue: issue, filename: 'dk.png')
+    create_list(:design, 2, :with_file, issue: issue)
 
     visit project_issue_path(project, issue)
 

ee/spec/features/projects/issues/design_management/user_views_design_spec.rb

@@ -5,12 +5,11 @@ describe 'User views issue designs', :js do
 
   set(:project) { create(:project_empty_repo, :public) }
   set(:issue) { create(:issue, project: project) }
+  set(:design) { create(:design, :with_file, issue: issue) }
 
   before do
     enable_design_management
 
-    create(:design, :with_file, issue: issue, filename: 'world.png')
-
     visit project_issue_path(project, issue)
 
     click_link 'Designs'
@@ -22,7 +21,7 @@ describe 'User views issue designs', :js do
     find('.js-design-list-item', match: :first).click
 
     page.within(find('.js-design-header')) do
-      expect(page).to have_content('world.png')
+      expect(page).to have_content(design.filename)
     end
 
     expect(page).to have_selector('.js-design-image')

ee/spec/features/projects/issues/design_management/user_views_designs_spec.rb

@@ -5,11 +5,10 @@ describe 'User views issue designs', :js do
 
   set(:project) { create(:project_empty_repo, :public) }
   set(:issue) { create(:issue, project: project) }
+  set(:design) { create(:design, :with_file, issue: issue) }
 
   before do
     enable_design_management
-
-    create(:design, :with_file, issue: issue)
   end
 
   context 'navigates from the issue view' do