Add test that opens the print preview dialog

At least this way we may find earlier gross mistakes like the one
causing bug 415340

CCBUGS: 415340

autotests/parttest.cpp

@@ -137,6 +137,7 @@ class PartTest
         void testTypewriterAnnotTool();
         void testJumpToPage();
         void testTabletProximityBehavior();
+        void testOpenPrintPreview();
 
     private:
         void simulateMouseSelection(double startX, double startY, double endX, double endY, QWidget *target);
@@ -1944,6 +1945,17 @@ void PartTest::testTabletProximityBehavior()
     QVERIFY( w->cursor().shape() == Qt::CursorShape( Qt::ArrowCursor ) );
 }
 
+void PartTest::testOpenPrintPreview()
+{
+    QVariantList dummyArgs;
+    Okular::Part part{ nullptr, nullptr, dummyArgs };
+    QVERIFY( openDocument( &part, QStringLiteral( KDESRCDIR "data/file1.pdf" ) ) );
+    part.widget()->show();
+    QVERIFY(QTest::qWaitForWindowExposed(part.widget()));
+    CloseDialogHelper closeDialogHelper( QDialogButtonBox::Close );
+    part.slotPrintPreview();
+}
+
 } // namespace Okular
 
 int main(int argc, char *argv[])