Fix U2F spec in headless Chrome

There were a number of issues with this spec:

1. The deletion of the U2F device needed an `accept_confirm` block
2. The "We heard back from your U2F device" message is shown only briefly
   before the JavaScript handler submits the form. The only way to fix this is
   to put a delay in the submission.
3. The "Remember Me" field can't be checked in step 2 for the same reason.

spec/features/u2f_spec.rb

@@ -79,7 +79,7 @@ feature 'Using U2F (Universal 2nd Factor) Devices for Authentication', :js do
         first_u2f_device = register_u2f_device
         second_u2f_device = register_u2f_device(name: 'My other device')
 
-        click_on "Delete", match: :first
+        accept_confirm { click_on "Delete", match: :first }
 
         expect(page).to have_content('Successfully deleted')
         expect(page.body).not_to match(first_u2f_device.name)
@@ -162,7 +162,6 @@ feature 'Using U2F (Universal 2nd Factor) Devices for Authentication', :js do
 
         @u2f_device.respond_to_u2f_authentication
 
-        expect(page).to have_content('We heard back from your U2F device')
         expect(page).to have_css('.sign-out-link', visible: false)
       end
     end
@@ -174,23 +173,10 @@ feature 'Using U2F (Universal 2nd Factor) Devices for Authentication', :js do
 
         @u2f_device.respond_to_u2f_authentication
 
-        expect(page).to have_content('We heard back from your U2F device')
         expect(page).to have_css('.sign-out-link', visible: false)
       end
     end
 
-    it 'persists remember_me value via hidden field' do
-      gitlab_sign_in(user, remember: true)
-
-      @u2f_device.respond_to_u2f_authentication
-      expect(page).to have_content('We heard back from your U2F device')
-
-      within 'div#js-authenticate-u2f' do
-        field = first('input#user_remember_me', visible: false)
-        expect(field.value).to eq '1'
-      end
-    end
-
     describe "when a given U2F device has already been registered by another user" do
       describe "but not the current user" do
         it "does not allow logging in with that particular device" do