Fix tests when configured with -DOPENSSL_USE_IPV6=0

Fix tests when configured with -DOPENSSL_USE_IPV6=0

In include/internal/sockets.h it says that you can disable IPv6, and only
defines OPENSSL_USE_IPV6 (to 0 or 1) if it's not already defined.

The codebase generally then checks `#if OPENSSL_USE_IPV6`.

However, test_bio_dgram uses `#if defined(OPENSSL_USE_IPV6)` which means it tries
to test IPv6 even if it's explicitly configured out with -DOPENSSL_USE_IPV6=0
(`#if defined(OPENSSL_USE_IPV6)` is always true).

This fixes that.

Change-Id: Ie1641c9dd654f27f3bdca186517df5599ad1059b

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19181)