Tests: Fix nullptr access in QmlDesigner tests

This is just a hot fix and will be replaced later on.

Change-Id: I63fb5dcadc965ca2e9ade67b0b748f7abb011aff
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

tests/auto/qml/qmldesigner/coretests/tst_testcore.h

@@ -29,6 +29,8 @@
 
 #include <QtTest>
 
+#include <extensionsystem/pluginmanager.h>
+
 
 class tst_TestCore : public QObject
 {
@@ -237,4 +239,7 @@ private slots:
     // QMLAnnotations
     void writeAnnotations();
     void readAnnotations();
+
+private:
+    ExtensionSystem::PluginManager pm; // FIXME remove
 };