Make benchmarks and viewer more accessible.

- Drop google_benchmark module dependency, wasn't really using it.
- Move benchmark models into test_data
- Rename benchmark => testspeed, make default output more like MJ testspeed.
- Update viewer to use new io functions and add get_data_into.
- Add "mjx-testspeed" and "mjx-viewer" bin scripts

PiperOrigin-RevId: 601277135
Change-Id: I782f666c58a96ac1a0a1c5ffc8ecf29d9fcd810f

mjx/pyproject.toml

@@ -36,6 +36,10 @@ dependencies = [
     "trimesh",
 ]
 
+[project.scripts]
+mjx-testspeed = "mujoco.mjx.testspeed:main"
+mjx-viewer = "mujoco.mjx.viewer:main"
+
 [project.urls]
 Homepage = "https://github.com/google-deepmind/mujoco/tree/main/mjx"
 Documentation = "https://mujoco.readthedocs.io/en/3.1.2"