test/testutil.c: Flush stdout when running tests

Because stdout is usually buffered and stderr isn't, error output
might get printed in one bunch and all the lines saying which test
failed all in one bunch, making it difficult to see exactly what error
output belongs to what test.  Flushing stdout makes sure the runner
output is displayed together with the corresponding error output.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3198)