Remove BIO_seek/BIO_tell from evp_test.c

BIO_seek and BIO_tell can cause problems with evp_test.c on some platforms.
Avoid them by using a temporary memory BIO to store key PEM data.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2183)