Created the Structure.test_bug_23295_ss_metadata_merge system test.
This is to catch bug #23295 (https://gna.org/bugs/?23295), the PDB secondary structure HELIX and
SHEET records not updated when merging molecules. This uses the '2BE6_secondary_structure.pdb'
structure file and 'test.py' relax script contents as the test, checking the HELIX and SHEET
records.