Fix list contents memory leak in wxAny unit test

Also document the need for using WX_CLEAR_LIST() when converting a
list-valued wxVariant to wxAny.

Note that we intentionally don't fix the problem by clearing the list
automatically, even if it could be done, because this would silently
break the existing code which does already clear the list -- and now
would attempt to clear it twice, with fatal consequences. Instead
document the existing behaviour and explain how to avoid memory leaks.

tests/any/anytest.cpp

@@ -652,6 +652,8 @@ void wxAnyTestCase::wxVariantConversions()
     CPPUNIT_ASSERT(variant.GetCount() == 2);
     CPPUNIT_ASSERT(variant[0].GetLong() == 15);
     CPPUNIT_ASSERT(variant[1].GetString() == "abc");
+    // Avoid the memory leak.
+    WX_CLEAR_LIST(wxAnyList, anyList);
 
     any = wxAny(vCustomType);
     CPPUNIT_ASSERT(wxANY_CHECK_TYPE(any, wxVariantData*));