Ib9e095430fa82be2327dcf7fd01ee4275b17415f introduced loops in "_run"
methods which have the side effect of events no longer being set.
Some tests are running until timeout expiration.

Test id                                                          Runtime
--------------------------------------------------------------  --------
tests.unit.test_gerrit.TestPolling.test_config_update            123.778
tests.unit.test_gerrit.TestPolling.test_post                     123.729
tests.unit.test_gerrit.TestChecksApi.test_new_patchset           123.635
tests.unit.test_gerrit.TestPolling.test_tag                      123.628

By changing the subclass method used to set event to one inside the
loop, notifications are restored.

Test id                                                          Runtime
--------------------------------------------------------------  --------
tests.unit.test_gerrit.TestPolling.test_config_update              5.246
tests.unit.test_gerrit.TestPolling.test_post                       5.252
tests.unit.test_gerrit.TestChecksApi.test_new_patchset             5.276
tests.unit.test_gerrit.TestPolling.test_tag                        5.247

Change-Id: I8c9ad6577d43b9b4866f83e4f8727615abf239c4