test: don't make it more complicated than necessary.

Original rationale behind using write in testutil was to accommodate
no-stdio builds. But is there evidence that no-stdio users would have
write or pre-defined meaning for file descriptors 1 and 2? Correct
answer is to provide way for no-stdio users who want to exercise
tests to plug in own BIO, not to make assumption that they have write.
And since we don't have to make such assumption, we can as well go
for simplest that works with standard library as specified by C
language standard.

Reviewed-by: Richard Levitte <levitte@openssl.org>