Change a few missing `login_as` to `gitlab_sign_in`

spec/features/help_pages_spec.rb

@@ -60,7 +60,7 @@ describe 'Help Pages', feature: true do
       allow_any_instance_of(ApplicationSetting).to receive(:help_text) { "My Custom Text" }
       allow_any_instance_of(ApplicationSetting).to receive(:help_page_support_url) { "http://example.com/help" }
 
-      login_as :user
+      gitlab_sign_in :user
       visit help_path
     end
 

spec/features/milestones/milestones_spec.rb

@@ -39,7 +39,7 @@ describe 'Milestone draggable', feature: true, js: true do
     end
 
     it 'assigns issue when it has been dragged to ongoing list' do
-      login_as(:admin)
+      gitlab_sign_in(:admin)
       create_and_drag_issue
 
       expect(@issue.reload.assignees).not_to be_empty

spec/features/projects/snippets_spec.rb

@@ -29,7 +29,7 @@ describe 'Project snippets', :js, feature: true do
 
     context 'when submitting a note' do
       before do
-        login_as :admin
+        gitlab_sign_in :admin
         visit namespace_project_snippet_path(project.namespace, project, snippets[0])
       end