[#76261684] Test we exit if RabbitMQ server closes connection

Test that `connection.HandleFatalError` is called if the connection is closed by the RabbitMQ server.

We simulate this using the RabbitMQ Management HTTP API to close the connection opened by `ProxyTCP`.

* * *

Also, prefer Gomega's `BeTrue()` and `BeFalse()` to `Equal()` when matching booleans.