Reset the stored logs at each notification test steps

This is an atempt to solve a new appearance of bug 1813147 where the
long test_instance_action notification sample test case logs too much.
This patch drops the logs of each successful test step to try to avoid
the overload of the logging system.

Change-Id: I0e5db9b2f423ab92ff93b6b3aada7d6b79a7abf6
Related-Bug: #1813147
(cherry picked from commit 9302771)

Author: Balazs Gibizer

nova/tests/fixtures.py

@@ -182,6 +182,12 @@ def fake_logging_setup(*args):
         self.useFixture(
             fixtures.MonkeyPatch('oslo_log.log.setup', fake_logging_setup))
 
+    def delete_stored_logs(self):
+        # NOTE(gibi): this depends on the internals of the fixtures.FakeLogger.
+        # This could be enhanced once the PR
+        # https://github.com/testing-cabal/fixtures/pull/42 merges
+        self.logger._output.truncate(0)
+
 
 class OutputStreamCapture(fixtures.Fixture):
     """Capture output streams during tests.

nova/tests/functional/notification_sample_tests/test_instance.py

@@ -407,6 +407,11 @@ def test_instance_action(self):
             # Ensure that instance is in active state after an action
             self._wait_for_state_change(self.admin_api, server, 'ACTIVE')
 
+            # if the test step did not raised then we consider the step as
+            # succeeded. We drop the logs to avoid causing subunit parser
+            # errors due to logging too much at the end of the test case.
+            self.stdlog.delete_stored_logs()
+
     def test_create_delete_server(self):
         fake_trusted_certs = ['cert-id-1', 'cert-id-2']
         server = self._boot_a_server(