[CI] Run loader tests from within loaders/ subdir

Author: stephane-caron

tox.ini

@@ -44,38 +44,44 @@ commands =
     mypy robot_descriptions --ignore-missing-imports
 
 [testenv:loader-idyntree]
+changedir = {toxinidir}/tests/loaders
 deps =
     idyntree >= 8.0.0
 commands =
-    python -m unittest tests/loaders/test_idyntree.py
+    python -m unittest test_idyntree.py
 
 [testenv:loader-mujoco]
+changedir = {toxinidir}/tests/loaders
 deps =
     mujoco >=2.2.1
 commands =
-    python -m unittest tests/loaders/test_mujoco.py
+    python -m unittest test_mujoco.py
 
 [testenv:loader-pinocchio]
+changedir = {toxinidir}/tests/loaders
 deps =
     pin >=2.6.14
 commands =
-    python -m unittest tests/loaders/test_pinocchio.py
+    python -m unittest test_pinocchio.py
 
 [testenv:loader-pybullet]
+changedir = {toxinidir}/tests/loaders
 deps =
     numpy >=1.23.4
     pybullet >=3.2.5
 commands =
-    python -m unittest tests/loaders/test_pybullet.py
+    python -m unittest test_pybullet.py
 
 [testenv:loader-robomeshcat]
+changedir = {toxinidir}/tests/loaders
 deps =
     pycollada >=0.6
     robomeshcat >= 1.0.4
 commands =
-    python -m unittest tests/loaders/test_robomeshcat.py
+    python -m unittest test_robomeshcat.py
 
 [testenv:loader-yourdfpy]
+changedir = {toxinidir}/tests/loaders
 deps =
     numpy >=1.23.4
     trimesh ~=3.16.0