tests/basic: Skip tests if PK algo is unavailable

* tests/basic.c (check_pubkey): Skip tests if the required public key
  algorithm is not available.

When libgcrypt is configured with a non-default --enable-pubkey-ciphers
option (e.g., without RSA), some of the tests cannot be run because the
algorithms are not supported. Check for unsupported algorithms using
gcry_pk_test_algo and print a message if they are not available.

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