tests/smime: include secret key material for Bob

This is taken from the same Internet Draft that test/smime/ca.crt
comes from.  See that draft for more details.
https://www.ietf.org/id/draft-dkg-lamps-samples-02.html#name-pkcs12-object-for-bob

We don't use it yet, but it will be used to decrypt other messages in
the test suite.

Note that we include it here with an empty passphrase, rather than
with the passphrase "bob" that it is supplied with in the I-D.  The
underlying cryptographic material is the same, but this way we can
import cleanly into gpgsm without having a passphrase set on it (gpgsm
converts an empty-string passphrase into no passphrase at all on
import).

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>