Test case should honor flag HAVE_WRITEV rather than WIN32 (#344)

Test cases where not honoring the HAVE_WRITEV flag but instead using WIN32 to determine if WRITEV was available or not.  This patch fixes that.

Fix By: Bulat Gaifullin (@bgaifullin)

test/ares-test-internal.cc

@@ -578,7 +578,7 @@ const struct ares_socket_functions VirtualizeIO::default_functions = {
 #endif
   },
   [](ares_socket_t s, const struct iovec * vec, int len, void *) {
-#ifdef _WIN32
+#ifndef HAVE_WRITEV
     return ares_writev(s, vec, len);
 #else
     return :: writev(s, vec, len);