tests: Add a benchmark for Elgamal.
* tests/benchmark.c (sample_public_elg_key_1024): New. (sample_private_elg_key_1024): New. (sample_public_elg_key_2048, sample_private_elg_key_2048): New. (sample_public_elg_key_3072, sample_private_elg_key_3072): New. (elg_bench): New. (main): Add elg_bench. Add commands "elg" and "public".