Restructure several .txt tests to unittest framework:
 - GeoInterface.txt -> test_geointerface.py
 - IterOps.txt -> test_iterops.py
 - Operations.txt -> test_operations.py
 - Persist.txt -> test_persist.py
 - Predicates.txt -> test_predicates.py
 - attribute-chains.txt -> merged to test_polygon.py
 - dimensions.txt -> merged to test_polygon.py
 - linemerge.txt -> test_linemerge.py
 - wkt_locale.txt -> test_locale.py
 - cascaded_union.txt -> test_union.py
 - Also merged test_unary_union.py -> test_union.py