Add custom attribute to custom element in test (gazebosim#1406)

Update test to confirm that gazebosim#54 is fixed.

Signed-off-by: Steve Peters <scpeters@openrobotics.org>

Author: scpeters

test/integration/custom_elems_attrs.sdf

@@ -10,7 +10,7 @@
         <child>L2</child>
       </joint>
 
-      <mysim:transmission name="simple_trans">
+      <mysim:transmission name="simple_trans" mysim:attr="custom_attribute">
         <mysim:type>transmission_interface/SimpleTransmission</mysim:type>
         <mysim:joint name="J1">
           <mysim:hardwareInterface>EffortJointInterface</mysim:hardwareInterface>

test/integration/sdf_custom.cc

@@ -139,7 +139,7 @@ TEST(SDFParser, ReloadCustomElements)
   ASSERT_NE(nullptr, customElem2);
 
   const std::string customElemStr =
-R"(<mysim:transmission name='simple_trans'>
+R"(<mysim:transmission name='simple_trans' mysim:attr='custom_attribute'>
   <mysim:type>transmission_interface/SimpleTransmission</mysim:type>
   <mysim:joint name='J1'>
     <mysim:hardwareInterface>EffortJointInterface</mysim:hardwareInterface>