perf-test/emacs: test showing longish threads

This particular thread takes about 100 times longer to display in
emacs than on the command line.

The parameter notmuch-show-max-text-part-size exists, but is currently
ineffective for this task because it only hides attachments, not
part 0.

In future commits, the parameter notmuch-show-{depth,height}-limit
will trigger a similar kind of hiding for bodies as
n-s-m-text-part-size already does for attachments.