Fix epic management flaky test

The fix consists of two parts:

- The first one is about waiting for the save button not to be
present after saving changes
- And the second one is about changing the expectation, asserting
in the epics index page that the deleted epic is not there, instead
of asserting on the message that the epic was deleted.

qa/qa/ee/page/group/epic/edit.rb

@@ -18,6 +18,9 @@ module QA
 
             def save_changes
               click_element :save_button
+              wait(max: 5, interval: 1, reload: false) do
+                has_no_element?(:save_button)
+              end
             end
 
             def delete_epic

qa/qa/ee/page/group/epic/index.rb

@@ -38,6 +38,14 @@ module QA
                 has_element?(:epic_title_text, text: title)
               end
             end
+
+            def has_no_epic?(title)
+              # First, check that the new epic button is present,
+              # meaning that the test is in the right page before
+              # checking that the epic is not there.
+              find_element(:new_epic_button)
+              has_no_element?(:epic_title_text, text: title)
+            end
           end
         end
       end

qa/qa/specs/features/ee/browser_ui/2_plan/epic/epics_management_spec.rb

@@ -15,21 +15,21 @@ module QA
 
         expect(page).to have_content(epic_title)
 
+        epic_edited_title = 'Epic edited via GUI'
         EE::Page::Group::Epic::Show.perform(&:click_edit_button)
         EE::Page::Group::Epic::Edit.perform do |edit|
-          edited_title = 'Epic edited via GUI'
-          edit.set_title(edited_title)
+          edit.set_title(epic_edited_title)
           edit.save_changes
 
-          expect(edit).to have_content(edited_title)
+          expect(edit).to have_content(epic_edited_title)
         end
 
         epic.visit!
         EE::Page::Group::Epic::Show.perform(&:click_edit_button)
-        EE::Page::Group::Epic::Edit.perform do |edit|
-          edit.delete_epic
+        EE::Page::Group::Epic::Edit.perform(&:delete_epic)
 
-          expect(edit).to have_content('The epic was successfully deleted')
+        EE::Page::Group::Epic::Index.perform do |index|
+          expect(index).to have_no_epic(epic_edited_title)
         end
       end