Corrects materials and objects imports in `check_terrain_importer.py` (#3411)

# Description

Update namespace for materials and objects in the terrain generator
test.

Fixes #3383

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [ ] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

source/isaaclab/test/terrains/check_terrain_importer.py

@@ -67,10 +67,11 @@ import numpy as np
 import isaacsim.core.utils.prims as prim_utils
 import omni.kit
 import omni.kit.commands
+from isaacsim.core.api.materials import PhysicsMaterial
+from isaacsim.core.api.materials.preview_surface import PreviewSurface
+from isaacsim.core.api.objects import DynamicSphere
 from isaacsim.core.api.simulation_context import SimulationContext
 from isaacsim.core.cloner import GridCloner
-from isaacsim.core.materials import PhysicsMaterial, PreviewSurface
-from isaacsim.core.objects import DynamicSphere
 from isaacsim.core.prims import RigidPrim, SingleGeometryPrim, SingleRigidPrim
 from isaacsim.core.utils.extensions import enable_extension
 from isaacsim.core.utils.viewports import set_camera_view