VMS: for testutil, make sure to use BIO_f_linebuffer

Without that, output comes one character per line.  It's the same
issue as has been observed before, this happens when using write()
on a record oriented stream (possibly unbuffered too).

This also uncovered a bug in BIO_f_linebuffer, where this would cause
an error:

    BIO_write(bio, "1\n", 1);

I.e. there's a \n just after the part of the string that we currently
ask to get written.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5352)