AutoTest: Fix and unify handling of group nodes

QtTest had been forgotten to handle correctly as well. So, it
was possible to have several levels of grouping which had not
been handled correctly.
By (current) definition we handle only one level of grouping,
so prohibit more for QtTest tree items as well.
Basically move the check into a separate function and use this
beforehand instead of creating a nullptr.

Change-Id: Icbf02eae67e89464f371eb349eecf2976636d05f
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/autotest/testtreeitem.cpp

@@ -302,6 +302,11 @@ bool TestTreeItem::isGroupNodeFor(const TestTreeItem *other) const
     return QFileInfo(other->filePath()).absolutePath() == filePath();
 }
 
+bool TestTreeItem::isGroupable() const
+{
+    return true;
+}
+
 QSet<QString> TestTreeItem::internalTargets() const
 {
     auto cppMM = CppTools::CppModelManager::instance();