Stub transaction_open? via allow instead of expect

In Rails 6.1 transaction_open? is called multiple times, so
we should either expect a particular number of calls or just
use allow and stub the return value.

According to the context of this spec, it's sufficient to
use allow in this case

spec/models/internal_id_spec.rb

@@ -100,7 +100,8 @@ RSpec.describe InternalId do
 
     context 'when executed outside of transaction' do
       it 'increments counter with in_transaction: "false"' do
-        expect(ActiveRecord::Base.connection).to receive(:transaction_open?) { false }
+        allow(ActiveRecord::Base.connection).to receive(:transaction_open?) { false }
+
         expect(InternalId::InternalIdGenerator.internal_id_transactions_total).to receive(:increment)
           .with(operation: :generate, usage: 'issues', in_transaction: 'false').and_call_original
 
@@ -158,7 +159,8 @@ RSpec.describe InternalId do
       let(:value) { 2 }
 
       it 'increments counter with in_transaction: "false"' do
-        expect(ActiveRecord::Base.connection).to receive(:transaction_open?) { false }
+        allow(ActiveRecord::Base.connection).to receive(:transaction_open?) { false }
+
         expect(InternalId::InternalIdGenerator.internal_id_transactions_total).to receive(:increment)
           .with(operation: :reset, usage: 'issues', in_transaction: 'false').and_call_original
 
@@ -228,7 +230,8 @@ RSpec.describe InternalId do
 
     context 'when executed outside of transaction' do
       it 'increments counter with in_transaction: "false"' do
-        expect(ActiveRecord::Base.connection).to receive(:transaction_open?) { false }
+        allow(ActiveRecord::Base.connection).to receive(:transaction_open?) { false }
+
         expect(InternalId::InternalIdGenerator.internal_id_transactions_total).to receive(:increment)
           .with(operation: :track_greatest, usage: 'issues', in_transaction: 'false').and_call_original