Allow flex sleeping

PiperOrigin-RevId: 917817500
Change-Id: Ia3bd5e52e7c2eaa3f70c81352d82c130b1d357f6

test/engine/testdata/sleep/scene.xml

@@ -0,0 +1,26 @@
+<mujoco>
+  <compiler meshdir="asset" texturedir="asset"/>
+
+  <statistic meansize=".05"/>
+
+  <visual>
+    <rgba haze="0.15 0.25 0.35 1"/>
+    <quality shadowsize="4096"/>
+    <map stiffness="700" shadowscale="0.5" fogstart="1" fogend="15" zfar="40" haze="1"/>
+  </visual>
+
+  <asset>
+    <texture type="skybox" builtin="gradient" rgb1="0.3 0.5 0.7" rgb2="0 0 0" width="512" height="512"/>
+    <texture name="texplane" type="2d" builtin="checker" rgb1=".2 .3 .4" rgb2=".1 0.15 0.2"
+      width="512" height="512" mark="cross" markrgb=".8 .8 .8"/>
+
+    <material name="matplane" reflectance="0.3" texture="texplane" texrepeat="10 10" texuniform="true"/>
+  </asset>
+
+  <worldbody>
+    <light diffuse=".4 .4 .4" specular="0.1 0.1 0.1" pos="0 0 2.0" dir="0 0 -1" castshadow="false"/>
+    <light directional="true" diffuse=".8 .8 .8" specular="0.2 0.2 0.2" pos="0 0 4" dir="0 0 -1"/>
+
+    <geom name="ground" type="plane" size="0 0 1" pos="0 0 0" quat="1 0 0 0" material="matplane" condim="1"/>
+  </worldbody>
+</mujoco>