module-transaction: Do not allow parallel conversations by default

Pam conversations per se may also run in parallel, but this implies that
the application supports this.

Since this normally not the case, do not create modules that may invoke
the pam conversations in parallel by default, adding a mutex to protect
such calls.

module-transaction_test.go

@@ -305,11 +305,10 @@ func Test_ModuleTransaction_InvokeHandler(t *testing.T) {
 	}
 }
 
-func Test_MockModuleTransaction(t *testing.T) {
+func testMockModuleTransaction(t *testing.T, mt *moduleTransaction) {
+	t.Helper()
 	t.Parallel()
 
-	mt, _ := NewModuleTransactionInvoker(nil).(*moduleTransaction)
-
 	tests := map[string]struct {
 		testFunc            func(mock *mockModuleTransaction) (any, error)
 		mockExpectations    mockModuleTransactionExpectations
@@ -914,3 +913,13 @@ func Test_MockModuleTransaction(t *testing.T) {
 		})
 	}
 }
+
+func Test_MockModuleTransaction(t *testing.T) {
+	mt, _ := NewModuleTransactionInvoker(nil).(*moduleTransaction)
+	testMockModuleTransaction(t, mt)
+}
+
+func Test_MockModuleTransactionParallelConv(t *testing.T) {
+	mt, _ := NewModuleTransactionInvokerParallelConv(nil).(*moduleTransaction)
+	testMockModuleTransaction(t, mt)
+}