t/mime: fix test to work w/o Email::MIME

t/mime: fix test to work w/o Email::MIME

Although the lazy loading changes were correct, the code
was still using PublicInbox::MIME as a fixed class.  Use
the `$cls' variable from the loop.

Favor ->subparts to ->parts, instead, too, since ->parts is
discouraged by the Email::MIME manpage and not implemented for
Eml.