Fix flaky import_export/group/tree_saver_spec.rb spec

The spec previously expected the last board in the serialized response
to be the one with the milestone, but the database may return the boards
in any order. We make the test more robust by searching for the specific
board ID that has the milestone.

Closes https://gitlab.com/gitlab-org/gitlab/issues/208763

ee/spec/lib/ee/gitlab/import_export/group/tree_saver_spec.rb

@@ -141,15 +141,12 @@ describe Gitlab::ImportExport::Group::TreeSaver do
 
     context 'when there are boards with predefined milestones' do
       let(:milestone) { Milestone::Upcoming }
-
-      before do
-        create(:board, group: group, milestone_id: milestone.id)
-      end
+      let!(:board_with_milestone) { create(:board, group: group, milestone_id: milestone.id) }
 
       it 'saves the milestone data' do
         expect_successful_save(group_tree_saver)
 
-        board_data = saved_group_json['boards'].last
+        board_data = saved_group_json['boards'].find { |board| board['id'] == board_with_milestone.id }
 
         expect(board_data).to include(
           'milestone_id' => milestone.id,
@@ -164,15 +161,12 @@ describe Gitlab::ImportExport::Group::TreeSaver do
 
     context 'when there are boards with persisted milestones' do
       let(:milestone) { create(:milestone) }
-
-      before do
-        create(:board, group: group, milestone: milestone)
-      end
+      let!(:board_with_milestone) { create(:board, group: group, milestone_id: milestone.id) }
 
       it 'saves the milestone data' do
         expect_successful_save(group_tree_saver)
 
-        board_data = saved_group_json['boards'].last
+        board_data = saved_group_json['boards'].find { |board| board['id'] == board_with_milestone.id }
 
         expect(board_data).to include(
           'milestone_id' => milestone.id,