Add reproduction test for expected bug behavior

This reproduces the consistent ordering bug behavior.

spec/workers/issue_placement_worker_spec.rb

@@ -37,12 +37,18 @@ RSpec.describe IssuePlacementWorker do
       described_class.new.perform(issue.id)
     end
 
-    it 'limits the sweep to QUERY_LIMIT records, and reschedules placement' do
+    context 'there are more than QUERY_LIMIT unplaced issues' do
+      let_it_be(:unplaced) do
         # Ensure there are more than N issues in this set
         n = described_class::QUERY_LIMIT
         create_list(:issue, n - 5, **unplaced)
+      end
+
+      it 'limits the sweep to QUERY_LIMIT records, and reschedules placement' do
+        expect(Issue).to receive(:move_nulls_to_end)
+                           .with(have_attributes(count: described_class::QUERY_LIMIT))
+                           .and_call_original
 
-      expect(Issue).to receive(:move_nulls_to_end).with(have_attributes(count: n)).and_call_original
         expect(described_class).to receive(:perform_async).with(issue_d.id)
 
         described_class.new.perform(issue.id)
@@ -50,6 +56,21 @@ RSpec.describe IssuePlacementWorker do
         expect(project.issues.where(relative_position: nil)).to exist
       end
 
+      it 'is eventually correct' do
+        prefix = project.issues.where.not(relative_position: nil).order(:relative_position).to_a
+        moved = project.issues.where.not(id: prefix.map(&:id))
+
+        described_class.new.perform(issue.id)
+
+        expect(project.issues.where(relative_position: nil)).to exist
+
+        described_class.new.perform(issue.id)
+
+        expect(project.issues.where(relative_position: nil)).not_to exist
+        expect(project.issues.order(:relative_position)).to eq(prefix + moved.order(:created_at, :id))
+      end
+    end
+
     it 'anticipates the failure to find the issue' do
       id = non_existing_record_id