Previously, the server was created with out an explicitly configured
address. This lead to it using any local address which will prefer
IPv6 (::0) if it's available. By contrast, the client was created
with a base URL that specified localhost as the host. This meant the
the client would prefer to connect to IPv4. Normally this wouldn't
cause a problem as nothing would be listening on the port in the IPv4
stack so the client would then connect to the server being tested
using the IPv6 stack. However, if another process was listening to the
port in the IPv4 stack, the client would connect to the wrong server.
This could lead to an unexpected 404 response (if the wrong server
was an HTTP server) or a hang if it was not.

There's a chance, although I think it's unlikely, that the problem
described above is the cause of gh-10569. I think it's unlikely as
the hang tracked by gh-10569 only occurs when running the WebFlux
endpoint integration tests using Reactor Netty. If it was the problem
described above, there's no reason that I can think of why we
wouldn't have also seen it with the Web MVC endpoint integration
tests.