Lib/test/output/test_StringIO is no longer necessary.
Also, added a test of the iterator protocol that's just been added to
StringIO's and cStringIO's.
cStringIO does not get it right (reported as SF bug #115531).
Added test for ValueError when write() is called on a closed StringIO
object. Commented out because cStringIO does not get it right
(reported as SF bug #115530).