auth: Better fix for the leak reported by LSAN in test-distributor_hh.cc

auth: Better fix for the leak reported by LSAN in test-distributor_hh.cc

This unit test is checking that we correctly throw when too many queries
are waiting in the distribution pipe, by making the mock backend slow
on purpose. Once the distributor has been restarted as expected, we need
to wait until the mock backend has processed all queued queries, otherwise
Leak Sanitizer will rightfully report a memory leak.

(cherry picked from commit 681cdfb76c06fbfe89e1ae606ac629e42829cb7c)