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".