tst_QTextStream::stillOpenWhenAtEnd(): use QTRY_VERIFY()

The test is flaky every now and then on the CI, so increase the timeout from
5s to 20s (a previous TRY_VERIFY precheck reported just shy of 10s waiting
time, doubling should be™ sufficient).

Amends the start of the public history.

Pick-to: 6.9 6.8 6.5 6.2 5.15
Change-Id: I8e64bf8ac1576c0ac71790a03936635016b3b2b7
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

tests/auto/corelib/serialization/qtextstream/tst_qtextstream.cpp

@@ -1205,7 +1205,7 @@ void tst_QTextStream::stillOpenWhenAtEnd()
 
     QTcpSocket socket;
     socket.connectToHost(QtNetworkSettings::imapServerName(), 143);
-    QVERIFY(socket.waitForReadyRead(5000));
+    QTRY_VERIFY_WITH_TIMEOUT(socket.bytesAvailable() > 0, 20000);
 
     QTextStream stream2(&socket);
     while (!stream2.readLine().isNull()) {}