starmath: close SfxObjectShell in unit test to stop temp file leakage

Change-Id: Iaf2df8a315b3e0c8cdd1f1a4a7d589682384ddaf

starmath/qa/cppunit/test_starmath.cxx

@@ -97,6 +97,7 @@ void Test::tearDown()
     delete m_pEditWindow;
     delete m_pSmCmdBoxWindow;
     delete m_pDispatcher;
+    m_xDocShRef->DoClose();
     m_xDocShRef.Clear();
 
     BootstrapFixture::tearDown();