In the following commit, we add a test that needs to block the CDC
generation publisher's loop twice. We allow it in this commit by
making handlers of the `cdc_generation_publisher_fiber` injection
share messages. From now on, unblocking every step of the loop will
require sending a new message from the test.
This change breaks the test already using the
`cdc_generation_publisher_fiber` injection, so we adjust the test.
f45aebeee2