[TEST] added sort by "_id" when score is the same to MultiMatchQueryTests#testEquivalence

A merge (and refresh) might rarely happen in the background between the two queries whose output is compared. It might then happen that two docs with same scores get returned by the two queries in a different order due to different lucene document id (which has changed in the meantime). To fix this we need to order by id when the score is the same, so that we can safely compare the output of the two queries (multimatch and dismax).