Created the State.test_bug_20480 system test to catch bug #20480 (https://gna.org/bugs/?20480).
This bug was reported by Stanislava Panova (https://gna.org/users/stacy). The file
test_suite/shared_data/saved_states/bug_20480.bz2 is the same as that attached to the bug report but
with all residues except for 9 and 10 deleted.
