Trivial test improvements

This commit reuses a variable instead of reevaluating the expression
and updates an outdated comment in the EVP test.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/7242)

(cherry picked from commit 523fcfb4)