Fix property test and add smw.dataItem.wikiPage.isKnown()

+ Due to Iba9946db, smw.dataItem.wikiPage is now able to tell if it
is a known page or not
+ Fix property test
+ Add a smw.api.query() toString() sanity check

Change-Id: I07c0a93b425132011456bb1d6b575598258114d4