tst_QTcpServer: convert QSKIP to blacklist

So we can gather statistics on whether it is still failing

Pick-to: 6.4 6.2
Change-Id: I1f4080f4d96f31ce2b689cda175af3a35563e232
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

tests/auto/network/socket/qtcpserver/BLACKLIST

@@ -16,3 +16,5 @@ windows-10
 [linkLocal]
 macos arm
 
+[addressReusable]
+linux # QTBUG-39985

tests/auto/network/socket/qtcpserver/tst_qtcpserver.cpp

@@ -578,9 +578,6 @@ void tst_QTcpServer::addressReusable()
 #if !QT_CONFIG(process)
     QSKIP("No qprocess support", SkipAll);
 #else
-#ifdef Q_OS_LINUX
-    QSKIP("The addressReusable test is unstable on Linux. See QTBUG-39985.");
-#endif
     QFETCH_GLOBAL(bool, setProxy);
     if (setProxy) {
 #ifndef QT_NO_NETWORKPROXY