tests: account for varying-size cryptographic signatures

GnuPG 2.1.16 is now injecting the full issuer fingerprint in its
signatures, which makes them about 32 octets larger when
ascii-armored.

This change in size means that the size of the MIME parts will vary
depending on the version of gpg that the user has installed.  at any
rate, the signature part should be non-zero (this is true for
basically any MIME part), so we just test for that instead of an exact
size.