ENH: Update ctkSettingsPanel tests for Qt5

Writing and reading an empty QVariant(QStringList()) to QSettings works as
expected in Qt5. Update the ctkSettingsPanel tests for Qt5.

Libs/Widgets/Testing/Cpp/ctkSettingsPanelTest.cpp

@@ -261,8 +261,13 @@ void ctkSettingsPanelTester::testEmptyQStringList()
   settingsPanel.setSettings(&settings2);
 
   QVariant listVal = settings2.value("list");
+#if QT_VERSION < QT_VERSION_CHECK(5,4,0)
   QCOMPARE(listVal.isValid(), false); // See issue #646
   QCOMPARE(listVal, QVariant()); // See issue #646
+#else
+  QCOMPARE(listVal.isValid(), true);
+  QCOMPARE(listVal, QVariant(QStringList()));
+#endif
   QCOMPARE(listVal.toStringList(), QStringList());
   QCOMPARE(settingsPanel.myPreviousPropertyValue("list").toStringList(), QStringList());
   QCOMPARE(settingsPanel.myDefaultPropertyValue("list").toStringList(), QStringList());

Libs/Widgets/Testing/Cpp/ctkSettingsPanelTest1.cpp

@@ -429,8 +429,13 @@ int TestStringList(ctkSettingsPanelForTest& settingsPanel)
   settingsPanel.setSettings(&settings2);
 
   listVal = settings2.value("key list");
+#if QT_VERSION < QT_VERSION_CHECK(5,4,0)
   CHECK_BOOL(listVal.isValid(), false); // Issue #646
   CHECK_QVARIANT(listVal, QVariant()); // Issue #646
+#else
+  CHECK_BOOL(listVal.isValid(), true);
+  CHECK_QVARIANT(listVal, QVariant(QStringList()));
+#endif
   CHECK_QSTRINGLIST(listVal.toStringList(), QStringList());
   CHECK_QSTRINGLIST(settingsPanel.myPreviousPropertyValue("key list").toStringList(), QStringList());
   CHECK_QSTRINGLIST(settingsPanel.myDefaultPropertyValue("key list").toStringList(), QStringList());