test:key: add a subkey to the testing key.

test:key: add a subkey to the testing key.

I updated `key.conf` to generate the appropriate key type, but
rerunning `key.sh` would create a whole new key.  I didn't want to
rewrite the old tests to use a new primary key fingerprint, so I added
a subkey to the old pgp-mime-test key by hand:

  $ gpg --expert --edit-key 4332B6E3
  ...
  gpg> addkey
  Please select what kind of key you want:
  ...
  (8) RSA (set your own capabilities)
  Your selection? 8
  ...
  Current allowed actions: Sign Encrypt
  ...
     (Q) Finished

  Your selection? Q
  What keysize do you want? (2048)
  Requested keysize is 2048 bits
  ...
  Key is valid for? (0) 0
  Is this correct? (y/N) y
  Really create? (y/N) y
  gpg> save
  $ gpg --export --armor -o test/key.txt 4332B6E3