QTest: make toString(QHostAddress) work

I don't know why this needs to be a template specialization instead of
overload, like all the other toString overloads. With GCC 7, it was
calling QTest::toString<QHostAddress>, which returns nullptr.

Change-Id: I638cf58bfa7b4e5fb386fffd14ea7add0eef8f87
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@qt.io>

src/testlib/qtest_network.h

@@ -67,7 +67,8 @@ namespace QTest
 /*!
     \internal
  */
-inline char *toString(const QHostAddress &addr)
+template<>
+inline char *toString<QHostAddress>(const QHostAddress &addr)
 {
     switch (addr.protocol()) {
     case QAbstractSocket::UnknownNetworkLayerProtocol: