Add elasticity plugins to xml_native_writer_test. Fixes #961.

PiperOrigin-RevId: 545454207
Change-Id: I4e93499eaa53e5bc8c8afa682986657cef3ac20f

test/fixture.h

@@ -19,6 +19,7 @@
 #include <absl/strings/string_view.h>
 #include <mujoco/mjdata.h>
 #include <mujoco/mjmodel.h>
+#include <mujoco/mujoco.h>
 
 namespace mujoco {
 
@@ -65,5 +66,29 @@ const std::string SaveAndReadXml(const mjModel* model);
 std::vector<mjtNum> GetCtrlNoise(const mjModel* m, int nsteps,
                                  mjtNum ctrlnoise = 0.01);
 
+// Installs elasticity plugins
+// TODO(quaglino): load all plugins with a macro
+class PluginTest : public MujocoTest {
+ public:
+  // load plugin library
+  PluginTest() : MujocoTest() {
+    #if defined(_WIN32) || defined(__CYGWIN__)
+      mj_loadPluginLibrary((
+        std::string(std::getenv("MUJOCO_PLUGIN_DIR")) +
+        std::string("\\elasticity.dll")).c_str());
+    #else
+      #if defined(__APPLE__)
+        mj_loadPluginLibrary((
+          std::string(std::getenv("MUJOCO_PLUGIN_DIR")) +
+          std::string("/libelasticity.dylib")).c_str());
+      #else
+        mj_loadPluginLibrary((
+          std::string(std::getenv("MUJOCO_PLUGIN_DIR")) +
+          std::string("/libelasticity.so")).c_str());
+      #endif
+    #endif
+  }
+};
+
 }  // namespace mujoco
 #endif  // MUJOCO_TEST_FIXTURE_H_