Log the possible socket error code for the flaky test

tst_QSocks5SocketEngine::simpleConnectToIMAP() is flaky. It may be
useful to log the socket error, to provide more info in case the
test fails again.

Change-Id: Ia5518dce13fd9da1fa5bfb3d5cf3a52a908b8698
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

tests/auto/network/socket/qsocks5socketengine/tst_qsocks5socketengine.cpp

@@ -309,7 +309,7 @@ void tst_QSocks5SocketEngine::simpleConnectToIMAP()
     QCOMPARE(socketDevice.peerAddress(), QtNetworkSettings::imapServerIp());
 
     // Wait for the greeting
-    QVERIFY(socketDevice.waitForRead());
+    QVERIFY2(socketDevice.waitForRead(), qPrintable("Socket error:" + socketDevice.errorString()));
 
     // Read the greeting
     qint64 available = socketDevice.bytesAvailable();