Destroy item sets in SdDialogsTest::tearDown

...as they apparently hold raw pointers to objects destroyed early, so only
destroying the item sets in the SdDialogsTest dtor caused use-after-free crashes
during shutdown.

Change-Id: Ibe3848c81a56fb0db4bf1fca0eab35836aab6a66

sd/qa/unit/dialogs-test.cxx

@@ -142,6 +142,9 @@ void SdDialogsTest::setUp()
 
 void SdDialogsTest::tearDown()
 {
+    mpSfxItemSetFromSdrObject.reset();
+    mpEmptySfxItemSet.reset();
+    mpEmptyFillStyleSfxItemSet.reset();
     mxComponent->dispose();
     ScreenshotTest::tearDown();
 }