tst_QNetworkInterface: enable IPv6 detection for Windows

This was disabled in d0d1d74033, I guess
accidentally, by a too-wide conditional. The change the same commit
applied to QtNetworkSettings didn't make the same mistake.

I am also opportunistically updating the conditional to QT_CONFIG (I
missed this in 9d4579c1cd) and adding the
Linux-specific check, as the AF_NETLINK implementation does not rely on
getifaddrs() or if_nametoindex().

Drive-by fix indentation.

Pick-to: 6.6
Change-Id: Ifa1111900d6945ea8e05fffd177ef8fcb11b4e1e
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

tests/auto/network/kernel/qnetworkinterface/CMakeLists.txt

@@ -15,6 +15,6 @@ qt_internal_add_test(tst_qnetworkinterface
     SOURCES
         tst_qnetworkinterface.cpp
     LIBRARIES
-        Qt::Network
+        Qt::NetworkPrivate
     QT_TEST_SERVER_LIST "apache2"
 )

tests/auto/network/kernel/qnetworkinterface/tst_qnetworkinterface.cpp

@@ -11,6 +11,8 @@
 #include <qudpsocket.h>
 #include "../../../network-settings.h"
 
+#include <private/qtnetwork-config_p.h>
+
 Q_DECLARE_METATYPE(QHostAddress)
 
 class tst_QNetworkInterface : public QObject
@@ -48,14 +50,13 @@ tst_QNetworkInterface::~tst_QNetworkInterface()
 
 bool tst_QNetworkInterface::isIPv6Working()
 {
- // Version without following cannot get IPV6 information
- #if !defined(QT_NO_GETIFADDRS) && !defined(QT_NO_IPV6IFNAME)
-     QUdpSocket socket;
-     socket.connectToHost(QHostAddress::LocalHostIPv6, 1234);
-     return socket.state() == QAbstractSocket::ConnectedState || socket.waitForConnected(100);
- #else
-     return false;
- #endif
+    // QNetworkInterface may be unable to detect IPv6 addresses even if they
+    // are there, due to limitations of the implementation.
+    if (QOperatingSystemVersion::currentType() == QOperatingSystemVersion::Windows ||
+            QT_CONFIG(linux_netlink) || (QT_CONFIG(getifaddrs) && QT_CONFIG(ipv6ifname))) {
+        return QtNetworkSettings::hasIPv6();
+    }
+    return false;
 }
 
 void tst_QNetworkInterface::initTestCase()