test: add crypto tests for signature verification and decryption

This adds a new "crypto" test script to the test suite to test
PGP/MIME signature verification and message decryption.  Included here
is a test GNUPGHOME with a test secret key (passwordless), and test
for:

  * signing/verification
  * signing/verification with full owner trust
  * verification with signer key unavailable
  * encryption/decryption
  * decryption failure with missing key
  * encryption/decryption + signing/verfifying
  * reply to encrypted message
  * verification of signature from revoked key

These tests are not expected to pass now, but will as crypto
functionality is included.