tst_qmessagehandler: force QProcess tests to log to stderr

These tests rely on reading output sent to stderr, so make
sure their output does go there.

Pick-to: 6.2 6.3
Change-Id: If62c073101c1d2e3d64f8cb2769d67f3b9fbeefb
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

tests/auto/corelib/global/qlogging/tst_qlogging.cpp

@@ -97,6 +97,7 @@ void tst_qmessagehandler::initTestCase()
             break;
         }
     }
+    m_baseEnvironment.prepend("QT_FORCE_STDERR_LOGGING=1");
 #endif // QT_CONFIG(process)
 }