Updates for some of the Structure.test_create_diff_tensor_pdb_* system tests.
This includes Structure.test_create_diff_tensor_pdb_ellipsoid,
Structure.test_create_diff_tensor_pdb_oblate, and Structure.test_create_diff_tensor_pdb_prolate.
The new simulated PDB truncation in the test data causes the PDB files created in these tests to be
slightly different.
