multipart/mixed message with no attachments.

test_one_part_in_a_multipart(): A test for the layout of a
multipart/mixed message with a single attachment.

test_seq_parts_in_a_multipart(): A test for the layout of a
multipart/mixed message with a single attachment that happens to be a
sequence of length one.

These tests ensure no regressions on the fix for SF bug #531966.

I will merge these into the standalone email package and Python 2.3
trunk separately.