Remove PluginTest and make MujocoTest load plugins.

MujocoTest now loads plugins from MUJOCO_PLUGIN_DIR if set.
PluginTest is removed; all tests use MujocoTest directly.
testspeed binary loads plugins from MUJOCO_PLUGIN_DIR.

This is in preparation for moving common asset format parsing (obj, msh, stl, etc.) where we will always want to load those plugins.

PiperOrigin-RevId: 874194428
Change-Id: Id90805a9ba5de4627911b56d8b9c4ab4e1b29310

test/engine/engine_plugin_test.cc

@@ -376,10 +376,10 @@ int RegisterNoAttributePlugin() {
   return mjp_registerPlugin(&plugin);
 }
 
-class EnginePluginTest : public PluginTest {
+class EnginePluginTest : public MujocoTest {
  public:
   // register all plugins
-  EnginePluginTest() : PluginTest() {
+  EnginePluginTest() : MujocoTest() {
     RegisterSensorPlugin();
 
     for (int i = 1; i <= kNumFakePlugins; ++i) {
@@ -466,7 +466,7 @@ TEST_F(MujocoTest, EmptyPluginDisallowed) {
   mj_deleteModel(m);
 }
 
-TEST_F(PluginTest, FirstPartyPlugins) {
+TEST_F(MujocoTest, FirstPartyPlugins) {
   EXPECT_THAT(mjp_pluginCount(), kNumTruePlugins);
 }