Created the Structure.test_bug_23294_multi_mol_automerge system test.
This is used to catch bug #2329 (https://gna.org/bugs/?23294), the automatic merging of PDB
molecules resulting in an IndexError. It reads in the 'in.pdb' PDB file attached to the bug report,
now named 'bug_23294_multi_mol_automerge.pdb', to show the IndexError. The test also checks the
structure.write_pdb user function to make sure that the output PDB file contains a single merged
molecule.
