Improve testing of elliptic curve validation

This involves:

- A directory of valid and invalid PEM-encoded curves.
  This is non-exhaustive and can be added to.
- A minor patch to 'openssl ecparam' to make it exit non-zero
  when curve validation fails.

- A test recipe is added in a separate commit.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2544)