Ensure specs on sorting of issues in API are deterministic on MySQL

MySQL could not have support for millisecond precision, depends on the MySQL version 
so we just create issues in different seconds in a deterministic way

CHANGELOG

@@ -18,6 +18,7 @@ v 8.12.0 (unreleased)
   - Use Search::GlobalService.new in the `GET /projects/search/:query` endpoint
   - Fix pagination on user snippets page
   - Fix sorting of issues in API
+  - Ensure specs on sorting of issues in API are deterministic on MySQL
   - Escape search term before passing it to Regexp.new !6241 (winniehell)
   - Fix pinned sidebar behavior in smaller viewports !6169
   - Change merge_error column from string to text type

spec/factories/issues.rb

 FactoryGirl.define do
+  sequence :issue_created_at do |n|
+    4.hours.ago + ( 2 * n ).seconds
+  end
+
   factory :issue do
     title
     author

spec/requests/api/issues_spec.rb

@@ -18,6 +18,7 @@ describe API::API, api: true  do
            project: project,
            state: :closed,
            milestone: milestone,
+           created_at: generate(:issue_created_at),
            updated_at: 3.hours.ago
   end
   let!(:confidential_issue) do
@@ -26,6 +27,7 @@ describe API::API, api: true  do
            project: project,
            author: author,
            assignee: assignee,
+           created_at: generate(:issue_created_at),
            updated_at: 2.hours.ago
   end
   let!(:issue) do
@@ -34,6 +36,7 @@ describe API::API, api: true  do
            assignee: user,
            project: project,
            milestone: milestone,
+           created_at: generate(:issue_created_at),
            updated_at: 1.hour.ago
   end
   let!(:label) do