Add basic site support to Mjcf SdfFileFormat plugin.

We will still need to tag these with a custom API schema to ease round-tripping but that will follow creation of such a schema.

PiperOrigin-RevId: 746421793
Change-Id: I2df2209b4bc90d763718fb4bea8158b8598755c6

test/experimental/usd/plugins/mjcf/fixture.h

@@ -29,12 +29,26 @@
 #define EXPECT_PRIM_VALID(stage, path) \
   EXPECT_TRUE((stage)->GetPrimAtPath(SdfPath(path)).IsValid());
 
+#define EXPECT_PRIM_IS_A(stage, path, type)                         \
+  {                                                                 \
+    EXPECT_TRUE((stage)->GetPrimAtPath(SdfPath(path)).IsA<type>()); \
+  }
+
 #define EXPECT_PRIM_KIND(stage, path, kind)                          \
   {                                                                  \
     pxr::TfToken prim_kind;                                          \
     pxr::UsdModelAPI::Get(stage, SdfPath(path)).GetKind(&prim_kind); \
     EXPECT_EQ(kind, prim_kind);                                      \
   }
+
+#define EXPECT_PRIM_PURPOSE(stage, path, purpose)    \
+  {                                                  \
+    pxr::TfToken prim_purpose;                       \
+    pxr::UsdGeomImageable::Get(stage, SdfPath(path)) \
+        .GetPurposeAttr()                            \
+        .Get(&prim_purpose);                         \
+    EXPECT_EQ(prim_purpose, purpose);                \
+  }
 namespace mujoco {
 
 pxr::SdfLayerRefPtr LoadLayer(const std::string& xml);