test_multi_subscriber test is flaky when unregistering topics

[sea-bass]

Description

• Library Version: Latest version humble and ros2 branches
• ROS Version: Humble and (sometimes) Jazzy?
• Platform / OS: Ubuntu 22.04 / 24.04

Steps To Reproduce
The test_multi_subscriber.py test is flaky in weird ways... This is specifically when trying to unregister a topic with multiple subscribers.

• On Humble, it fails consistently
• On Jazzy, it seems flaky
• Have not yet seen any failures on Iron or Rolling

The only related change that has gone in lately is #948, which could be the cause. But either way, nondeterminism is bad.

Expected Behavior
The test works deterministically.

Actual Behavior

              multi.unregister()
      >       self.assertFalse(is_topic_subscribed(self.node, topic))
      E       AssertionError: True is not false
      
      ../../src/c1dv9q94shc/rosbridge_suite/rosbridge_library/test/internal/subscribers/test_multi_subscriber.py:96: AssertionError