tst_QNetworkReply: cleanup changed state from timeout test

It was not setting the value _back_ to 0 when the function ended
and this caused qtbug68821proxyError to fail fairly consistently
on Windows when it was running in the same run.

qtbug68821proxyError was always succeeding when ran by itself so
it was quite odd.

Change-Id: Ifa4982f1b10128674081136a30bdab4b0ce7004a
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Juha Vuolle <juha.vuolle@qt.io>
(cherry picked from commit 5e102a792c4ee0db180defe480cb96c197871508)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/auto/network/access/qnetworkreply/tst_qnetworkreply.cpp

@@ -9931,6 +9931,7 @@ void tst_QNetworkReply::requestWithTimeout()
     // Manager instance remains between case runs => always reset it's transferTimeout to
     // ensure setting its transferTimeout in this case has effect
     manager.setTransferTimeout(0ms);
+    auto cleanup = qScopeGuard([this] { manager.setTransferTimeout(0ms); });
 
     MiniHttpServer server(tst_QNetworkReply::httpEmpty200Response, false);
     server.stopTransfer = true;