(tests) Fix memory leak in integration test

Found by coincidence when working on other code; I realized that the array allocated in the StringHashSet::values_wrapper() method would never be freed.

An interesting aspect of this is that this hasn't been caught by our Valgrind safeguards. I think that the tests exercising this method are executed via Valgrind in fact, but it still doesn't seem to be detected - perhaps because of the Valgrind configuation we use? Either way, this should of course be fixed.