Fix Mock backend test parametrization

Exclude the Mock backend case-insensitively from cache tests that only apply to solver-backed articulation implementations.

Author: AntoineRichard

source/isaaclab/test/assets/test_articulation_iface.py

@@ -237,7 +237,9 @@ def test_find_joints_single(self, backend, num_instances, num_joints, num_bodies
 # ---------------------------------------------------------------------------
 
 
-_non_mock_backends = pytest.mark.parametrize("backend", [b for b in BACKENDS if b != "mock"], indirect=False)
+_non_mock_backends = pytest.mark.parametrize(
+    "backend", [backend for backend in BACKENDS if backend.lower() != "mock"], indirect=False
+)
 
 
 class TestResolveMatchingNamesCache: