Improve island-constraint testing model.

PiperOrigin-RevId: 561931744
Change-Id: I2826d4c5b987cc6edd761026f6734fe40fb31278

test/engine/testdata/island/island_efc.xml

@@ -1,10 +1,10 @@
 <!--
   test model for constraint islanding:
-- one equality
-- one limit
-- one friction
-- one unconstrained dof
-- two islands, one with non-contiguous dofs
+- 1 equality
+- 1 limit
+- 2 friction constraints
+- 1 unconstrained dof
+- 3 islands, 2 with non-contiguous dofs
 -->
 
 <mujoco>
@@ -16,15 +16,22 @@
     <geom size=".1"/>
   </default>
 
+  <visual>
+    <headlight diffuse=".9 .9 .9"/>
+  </visual>
+
   <worldbody>
+
     <body>
       <joint type="slide" axis="0 0 1" range="0 1" limited="true"/>
       <geom/>
     </body>
+
     <body pos=".25 0 0">
       <joint type="slide" axis="1 0 0"/>
       <geom/>
     </body>
+
     <body pos="0 0 0.25">
       <joint type="slide" axis="0 0 1"/>
       <geom/>
@@ -37,14 +44,26 @@
         </body>
       </body>
     </body>
+
+    <body pos=".5 0 0">
+      <joint type="slide" axis="0 0 1" frictionloss="15"/>
+      <geom type="box" size=".08 .08 .02" euler="0 10 0"/>
+    </body>
+
     <body pos="-.5 0 0">
       <joint axis="0 1 0" frictionloss=".01"/>
       <geom type="capsule" size="0.03" fromto="0 0 0 -.2 0 0"/>
     </body>
+
     <body pos="0 0 .5">
       <joint name="hinge2" axis="0 1 0"/>
       <geom type="box" size=".08 .02 .08"/>
     </body>
+
+    <body pos=".5 0 .1">
+      <freejoint/>
+      <geom type="box" size=".03 .03 .03" pos="0.01 0.01 0.01"/>
+    </body>
   </worldbody>
 
   <equality>