test: overhaul multipart test to test for improved message/rfc822 handling

The main goal of this overhaul is to define how message/rfc822 parts
should be handled.  message/rfc822 parts should be output in a similar
fashion to the outer message, including some subset of the rfc822
headers.  The following decisions about formatting of message/rfc822
parts were made:

The format and content of message/rfc822 parts shall be as similar as
possible to that of full messages.  In particular, for formatted
outputs, the "content" of rfc822 part output should include "headers"
and "body" fields).

The "body" field shall include the body of the message.

The "headers" field shall include the following headers, since these
are the ones available from the GMimeMessage:

  "From"
  "To"
  "Cc"
  "Subject"
  "Date"

However, for the case of --format=raw the raw rfc822 should be output,
including all headers.

A subset of relevant headers shall be output in reply.

The test embedded rfc822 message is also modified to be itself
multipart, so we can more fully test how all sub parts of the message
part are output.

Note added by Committer:

Currently, expect one test (--format=raw --part=3, rfc822 part) to fail.