Remove QSKIP and bring the test back to business

on macOS, where it was skipped but where it now seems to be stable/work.

Task-number: QTBUG-39983
Change-Id: I100a57f23b43074ebacc012be247d92acc6ae336
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

tests/auto/corelib/io/qiodevice/tst_qiodevice.cpp

@@ -209,9 +209,6 @@ void tst_QIODevice::read_QByteArray()
 //--------------------------------------------------------------------
 void tst_QIODevice::unget()
 {
-#if defined(Q_OS_MAC)
-    QSKIP("The unget network test is unstable on Mac. See QTBUG-39983.");
-#endif
     QBuffer buffer;
     buffer.open(QBuffer::ReadWrite);
     buffer.write("ZXCV");