tests/benchmark: Skip unavailable PK algorithms

* tests/benchmark.c (cipher_bench, rsa_bench, elg_bench, dsa_bench,
  ecc_bench): Do not run benchmarks for PK algorithms that are not
  supported in the current build.

When libgcrypt is configured with a non-default --enable-pubkey-ciphers
option, some of the benchmarks cannot be run because the algorithms are
not supported. Disable those using preprocessor macros, but add explicit
uses of the function parameters to avoid compiler warnings.

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