tests/pubkey: Skip tests if PK algo is unavailable

* tests/pubkey.c (check_run, main): Skip tests for unavailable
  algorithms.

When libgcrypt is configured with a non-default --enable-pubkey-ciphers
option, some of the tests cannot be run because the algorithms are not
supported. Skip unsupported tests using preprocessor macros in this
case.

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