test: Add a test of "notmuch show" with a multipart message

This tests "notmuch show" with both --format=text and --format=json on
a message with some non-trivial MIME multipart nesting, (multiple parts
within a multipart/mixed part which is within a multipart/signed part).

The test captures the current behavior (where only the leaf nodes of
the MIME structure are emitted as a flat list---the multipart parts
are effectively ignored). We plan to soon change the json output at
least to emit an actual hierarchy matching the MIME structure, (at
which point we will update this test).