tests/keygen.c: Skip unavailable PK algorithms

* tests/keygen.c (show_mpi, check_rsa_keys, check_elg_keys,
  check_dsa_keys, check_generated_ecc_key, check_ecc_keys): Skip tests
  if the required public key algorithm is not available.

When libgcrypt is configured with a non-default --enable-pubkey-ciphers
option, some of the keygen tests cannot be run because the algorithms
are not supported. Disable those using preprocessor macros.

• GnuPG-bug-id: T6048
• Signed-off-by: Clemens Lang <cllang@redhat.com>