Fix specs for MySQL

spec/lib/gitlab/background_migration/populate_untracked_uploads_spec.rb

@@ -9,7 +9,10 @@ describe Gitlab::BackgroundMigration::PopulateUntrackedUploads, :sidekiq do
   let!(:uploads) { described_class::Upload }
 
   before do
+    DatabaseCleaner.clean
+    drop_temp_table_if_exists
     ensure_temporary_tracking_table_exists
+    uploads.delete_all
   end
 
   after(:all) do

spec/lib/gitlab/background_migration/prepare_untracked_uploads_spec.rb

@@ -18,6 +18,8 @@ describe Gitlab::BackgroundMigration::PrepareUntrackedUploads, :sidekiq do
   end
 
   before do
+    DatabaseCleaner.clean
+
     drop_temp_table_if_exists
   end