Fix up StringIO use in tests for py3

Some spots in this file were already referencing six.StringIO, but
some were still using StringIO.StringIO, which does not work on
py3. This patch just makes them all use six.StringIO and removes the
now-unused (and wrong, on py3) import.

Change-Id: I3c7311c3983f4eb409eedb6f85ede6ffe4059e63
Merge "Fix up StringIO use in tests for py3"