Hide global qtlogging.ini from autotest

Do not let a global qtlogging.ini interfere with an autotest.
This works around an issue on Ubuntu 17.10

Task-number: QTBUG-67385
Change-Id: I0d02835eb7a561b43fe0b98f4383c170c6d51303
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Joni Jäntti <joni.jantti@qt.io>

tests/auto/corelib/io/qloggingcategory/tst_qloggingcategory.cpp

@@ -203,6 +203,7 @@ private:
 private slots:
     void initTestCase()
     {
+        qputenv("XDG_CONFIG_DIRS", "/does/not/exist");
         qputenv("QT_MESSAGE_PATTERN", QByteArray("%{category}: %{type},%{message}"));
         oldMessageHandler = qInstallMessageHandler(myCustomMessageHandler);
         // Create configuration

tests/auto/corelib/io/qloggingregistry/tst_qloggingregistry.cpp

@@ -50,6 +50,7 @@ private slots:
     {
         // ensure a clean environment
         QStandardPaths::setTestModeEnabled(true);
+        qputenv("XDG_CONFIG_DIRS", "/does/not/exist");
         qunsetenv("QT_LOGGING_CONF");
         qunsetenv("QT_LOGGING_RULES");
     }