Cleanup mttest.c : modernise output

Construct bio_err and bio_stdout from file handles instead of FILE
pointers, since the latter might not be implemented (when OPENSSL_NO_STDIO
is defined).
Convert all output to use BIO_printf.
Change lh_foo to lh_SSL_SESSION_foo.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(cherry picked from commit bb8abd67)

Conflicts:
	crypto/threads/mttest.c