EVP_PKEY support for evp_test

Add two new keywords "PublicKey" and "PrivateKey". These will load a key
in PEM format from the lines immediately following the keyword and assign
it a name according to the value. These will be used later for public and
private key testing operations.

Add tests for Sign, Verify, VerifyRecover and Decrypt.
Reviewed-by: Richard Levitte <levitte@openssl.org>