add spec for validating membership restrictions of existing cols

the prior implementation of this validator ignored and overwrote existing
collections. the forms don't expand existing `member_of_collection_ids` into
attributes for submission, so it's important that we validate membership based
both on the existing ids AND any new submitted attributes.

the newer implementation handles this case, so this adds a regression test for
the requirement.

spec/validators/hyrax/collection_membership_validator_spec.rb

@@ -81,10 +81,31 @@
             end
           end
 
-          context 'and work is in another collection that IS posing a conflict' do
+          context 'and work is already ni collections with membership restriction conflicts' do
             let(:work) { FactoryBot.build(:hyrax_work, member_of_collection_ids: [sm_col1.id]) }
             let(:sm_col1) { FactoryBot.valkyrie_create(:hyrax_collection, collection_type_gid: single_mem_col_type.to_global_id) }
             let(:sm_col2) { FactoryBot.valkyrie_create(:hyrax_collection, collection_type_gid: single_mem_col_type.to_global_id) }
+
+            let(:mem_of_cols_attrs) do
+              { "0" => { "id" => sm_col2.id.to_s, "_destroy" => "false" } }
+            end
+
+            it 'adds validation errors' do
+              validator.validate(form)
+
+              expect(form.errors)
+                .to contain_exactly(
+                      start_with('Member of collection ids Error: ' \
+                                 'You have specified more than one of ' \
+                                 'the same single-membership collection type')
+                    )
+            end
+          end
+
+          context 'and work is added to collections with membership restriction conflicts' do
+            let(:work) { FactoryBot.build(:hyrax_work) }
+            let(:sm_col1) { FactoryBot.valkyrie_create(:hyrax_collection, collection_type_gid: single_mem_col_type.to_global_id) }
+            let(:sm_col2) { FactoryBot.valkyrie_create(:hyrax_collection, collection_type_gid: single_mem_col_type.to_global_id) }
             let(:mem_of_cols_attrs) do
               { "0" => { "id" => sm_col1.id.to_s, "_destroy" => "false" },
                 "1" => { "id" => sm_col2.id.to_s, "_destroy" => "false" } }