Use procedural model creation in plugin tests.

This will be useful for simplifying the mjSpec API for plugins.

PiperOrigin-RevId: 772505558
Change-Id: I8eb57d7e129e0f6430fb4c610a543e0dc0f3b295

python/mujoco/specs_test.py

@@ -861,7 +861,7 @@ class SpecsTest(absltest.TestCase):
 
     body = spec.worldbody.add_body()
     body.plugin = plugin
-    body.plugin.plugin_name = 'mujoco.elasticity.cable'
+    body.plugin.name = 'instance_name'
     body.plugin.active = True
 
     geom = body.add_geom()