test: optimize test_fast_shutdown with event-based synchronization

Replace sleep-based timing with proper Event synchronization:
- Remove executor_init with 0.5s sleep
- Replace time.sleep(0.5) with connection_created.wait()
- Replace time.sleep(3) with executor.shutdown(wait=True)
- Reduce iterations from 20 to 3 (deterministic with proper sync)
- Add explicit assertions for shutdown state

Performance improvement:
- Before: 70.13s call tests/unit/test_host_connection_pool.py::HostConnectionTests::test_fast_shutdown
- After: 0.01s call tests/unit/test_host_connection_pool.py::HostConnectionTests::test_fast_shutdown

Signed-off-by: Yaniv Kaul <yaniv.kaul@scylladb.com>

Author: mykaul

tests/unit/test_host_connection_pool.py

@@ -239,7 +239,8 @@ class MockSession(MagicMock):
             def __init__(self, *args, **kwargs):
                 super(MockSession, self).__init__(*args, **kwargs)
                 self.cluster = MagicMock()
-                self.cluster.executor = ThreadPoolExecutor(max_workers=2, initializer=self.executor_init)
+                self.connection_created = Event()
+                self.cluster.executor = ThreadPoolExecutor(max_workers=2)
                 self.cluster.signal_connection_failure = lambda *args, **kwargs: False
                 self.cluster.connection_factory = self.mock_connection_factory
                 self.connection_counter = 0
@@ -259,23 +260,30 @@ def mock_connection_factory(self, *args, **kwargs):
                                                                     partitioner="", sharding_algorithm="", sharding_ignore_msb=0,
                                                                     shard_aware_port="", shard_aware_port_ssl=""))
                 self.connection_counter += 1
+                self.connection_created.set()
 
                 return connection
 
-            def executor_init(self, *args):
-                time.sleep(0.5)
-                LOGGER.info("Future start: %s", args)
-
-        for attempt_num in range(20):
-            LOGGER.info("Testing fast shutdown %d / 20 times", attempt_num + 1)
+        for attempt_num in range(3):
+            LOGGER.info("Testing fast shutdown %d / 3 times", attempt_num + 1)
             host = MagicMock()
             host.endpoint = "1.2.3.4"
-            session = self.make_session()
+            session = MockSession()
 
             pool = HostConnection(host=host, host_distance=HostDistance.REMOTE, session=session)
             LOGGER.info("Initialized pool %s", pool)
+
+            # Wait for initial connection to be created (with timeout)
+            if not session.connection_created.wait(timeout=2.0):
+                pytest.fail("Initial connection failed to be created within 2 seconds")
+
             LOGGER.info("Connections: %s", pool._connections)
-            time.sleep(0.5)
+
+            # Shutdown the pool
             pool.shutdown()
-            time.sleep(3)
-            session.cluster.executor.shutdown()
+
+            # Verify pool is shut down
+            assert pool.is_shutdown, "Pool should be marked as shutdown"
+
+            # Cleanup executor with proper wait
+            session.cluster.executor.shutdown(wait=True)