Created the Structure.test_bug_21522_master_record_atom_count system test.
This is designed to catch bug #21522 (https://gna.org/bugs/?21522), the structure.write_pdb user
function creating an incorrect MASTER record. This hence also catches bug #21520
(https://gna.org/bugs/?21520), the failure of the structure.write_pdb user function when creating
the MASTER record due to too many ATOM and HETATM records being present.
The test simply creates to structural models, adds one atom, and writes out a PDB file, checking its
contents.
