Enhance OpenSSL::Test::cmdstr to give cmd string variants

Within OpenSSL::Test, all commands end up existing in two variants,
one that has redirections that are needed internally to work well
together with the test harness, and one without those redirections.

Depending on what the result is going to be used for, the caller may
want one for or the other, so we give them the possibility.

Reviewed-by: Rich Salz <rsalz@openssl.org>