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_ray_test.cc

@@ -82,7 +82,7 @@ using ::testing::DoubleNear;
 using ::testing::ElementsAre;
 using ::testing::NotNull;
 using ::testing::Pointwise;
-using RayTest = PluginTest;
+using RayTest = MujocoTest;
 
 TEST_F(RayTest, NoExclusions) {
   char error[1024];