tests: Add ECC key generation tests.

* tests/keygen.c (check_generated_ecc_key): New.
(check_ecc_keys): New.
(main): Call simple ECC checks.

Although benchmark.c does ECC key generation, it does not call
gcry_pk_testkey. We do it here. Note that we should do some more
checks, for example checking that all curve parameters are available
and match the specs.