Bug fix in openeye.oemols_to_ffxml

openeye.oemols_to_ffxml(molecules) returns list all_trajectories, which should contain a trajectory for each input molecule

Previously:
len(all_trajectories) == 1 regardless of the length of input molecule list

Updates:
openeye.oemols_to_ffxml (one line fix, append all_trajectories within each iteration of the molecules loop)
test_openeye.test_ffxml_simulation: added benzene to input molecule list, ran the test simulation for each input molecule