Make sure that apps/openssl prefixes its output with '# ' during tests

The reason to do this is that some output might start with an 'ok',
which TAP catches and takes for TAP output.  The TAP compatible way is
to make all output it shouldn't catch look like comments.

We do this by setting the environment variable HARNESS_OSSL_PREFIX
during tests.  When that is set, apps/openssl uses BIO_f_linebuffer
and sets its prefix to the content of that environment variable.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5224)