Report grammar rules that did not produce any valid queries

Since the RQG masks syntax and semantic errors, it is sometimes difficult to spot that a given grammar rule produces only defective queries.

In order to detect such situations, when --debug is specified, for each generated query we must also produce a list of participating grammar rules. Then , we tag each rule with the status code of the query. At the end of the test, we print all the rules that never got a STATUS_OK tag.