Updated test_atom_arrangement.py

[AbdullahKazi500]

Issue #, if available:
fixes #969
Description of changes:
test_add_chaining: Verifies the add method supports method chaining.
test_iteration: Ensures iteration over AtomArrangement returns the correct coordinates.
test_coordinate_list: Validates the coordinate_list method returns the correct coordinate lists.
test_discretize: Parametrized test that checks the discretize method for various resolutions.
test_invalid_discretization_properties: Expects a DiscretizationError when invalid properties are provided.
Testing done:
added test

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[peterkomar-aws]

Please consolidate the work to this PR: #989