test/duplicate-mid: check for subject with notmuch-show

In [1] Mark showed that the the current code (d7a49e81) is not
consistent in it's handling of subjects of messages with duplicate
message-ids (or in notmuch-speak, of messages with multiple files).
notmuch-search uses indexing order and explicitedly preserves the
first. notmuch-show (apparently) uses alphabetical (or at least xapian
term order) of filenames. In a perfect world we would probably report
all subjects in the json output; at the very least we should be
consistent.

[1]: id:87378dny3d.fsf@qmul.ac.uk