Add tests for S/MIME PKCS#7 messages

[Daniel Kahn Gillmor]

S/MIME messages that use PKCS#7 are not currently well-handled by
notmuch.

This series introduces a set of tests that cover such messages, most
of which are initially broken.  A future (shorter) series will resolve
these tests.

Some S/MIME messages *are* handled correctly by notmuch already: in
particular, multipart/signed messages where the detached signature is
a PKCS#7 signature object (as opposed to one-part S/MIME SignedData
messages) are already working.

There should be no functional change to notmuch from applying this
series, just a more complete test suite.

Comments and feedback welcome, as always!

Regards,

        --dkg