Avoid persisting subject for faster MR association tests

ee/spec/models/merge_request_spec.rb

@@ -16,6 +16,8 @@ RSpec.describe MergeRequest do
   subject(:merge_request) { create(:merge_request, source_project: project, target_project: project) }
 
   describe 'associations' do
+    subject { build_stubbed(:merge_request) }
+
     it { is_expected.to have_many(:approvals).dependent(:delete_all) }
     it { is_expected.to have_many(:approvers).dependent(:delete_all) }
     it { is_expected.to have_many(:approver_users).through(:approvers) }

spec/models/merge_request_spec.rb

@@ -12,6 +12,8 @@ RSpec.describe MergeRequest do
   subject { create(:merge_request) }
 
   describe 'associations' do
+    subject { build_stubbed(:merge_request) }
+
     it { is_expected.to belong_to(:target_project).class_name('Project') }
     it { is_expected.to belong_to(:source_project).class_name('Project') }
     it { is_expected.to belong_to(:merge_user).class_name("User") }