Don't incorrectly skip the multiblock test

Don't incorrectly skip the multiblock test

sslapitest has a test for multiblock writes. Since multiblock writing is
only available on some platforms the multiblock test checks whether we are
on such a platform first, and skips the test if we are not. Unfortunately
a bug in the check meant that we always skipped the test regardless of the
platform.

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18983)

(cherry picked from commit 7c82a7a8f3a66f47f727d31691d6298d88ed158b)