Update test_utils_trajectories.py

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[ ] I have read the CONTRIBUTING document.
[ ] My code follows the code style of this project (see .code-style.md).
[x] All new and existing tests passed (see .testing.md).
[ ] I have added tests to cover my changes.
[ ] My change requires a change to the documentation.
[ ] I have updated the documentation accordingly (see .documentation.md).

Increased coverage of object_for_array function