Attempt to fix the flaky TestTriggererJob test (#34075)

As documented in #33323, we have frequent failures of the flaky
triggerer job tests.

The flaky failures are about some errors when we close all the
sessions in teardown of the test. It turns out that the tests
had side-effect - they have not waited for the TriggererJob thread
to complete, they merely marked them to be stopped, but they have
not waited for those to complete - this is quite plausible
explanation of the flaky test failures - since those threads have
1 second sleep, it's more than likely that the session has been
created and used by the thread while the teardown has been attempting
to close all the sessions.

This side effect could also have an effect for other tests that
were run after - because in a busy test run machine, the side
effects could propagate further than just to the teardown, so it
could also explain why sometimes (very rarely) other job tests
failed with similar errors.

The fix is to join the runner after marking it to be stopped.

Fixes: #33323 (Hopefully)

tests/jobs/test_triggerer_job.py

@@ -143,6 +143,8 @@ def __init__(self, password, **kwargs):
     finally:
         # We always have to stop the runner
         triggerer_job_runner.trigger_runner.stop = True
+        triggerer_job_runner.trigger_runner.join(30)
+
     # Since we have now an in-memory process of forwarding the logs to stdout,
     # give it more time for the trigger event to write the log.
     time.sleep(0.5)
@@ -257,6 +259,7 @@ def test_trigger_lifecycle(session):
     finally:
         # We always have to stop the runner
         job_runner.trigger_runner.stop = True
+        job_runner.trigger_runner.join(30)
 
 
 class TestTriggerRunner:
@@ -408,7 +411,7 @@ def handle_events(self):
             pytest.fail("did not observe 2 loops in the runner thread")
     finally:
         job_runner.trigger_runner.stop = True
-        job_runner.trigger_runner.join()
+        job_runner.trigger_runner.join(30)
         thread.join()
     instances = path.read_text().splitlines()
     assert len(instances) == 1
@@ -514,7 +517,7 @@ async def create_triggers(self):
     finally:
         job_runner.trigger_runner.stop = True
         # with suppress(MockTriggerException):
-        job_runner.trigger_runner.join()
+        job_runner.trigger_runner.join(30)
         thread.join()
 
 
@@ -545,6 +548,7 @@ def test_trigger_firing(session):
     finally:
         # We always have to stop the runner
         job_runner.trigger_runner.stop = True
+        job_runner.trigger_runner.join(30)
 
 
 def test_trigger_failing(session):
@@ -578,6 +582,7 @@ def test_trigger_failing(session):
     finally:
         # We always have to stop the runner
         job_runner.trigger_runner.stop = True
+        job_runner.trigger_runner.join(30)
 
 
 def test_trigger_cleanup(session):