cohttp-eio: update tests for new Eio 1.4 getaddrinfo exception

[talex5]

Trying to connect to "http://foo.invalid" now raises Eio.Io Net Address_lookup_failed, not Failure.

Before (getaddrinfo returns [] and cohttp-eio raises an exception):

test_forward_proxy.exe: [INFO] GET http://foo.invalid
[failure] failed to resolve hostname

With Eio 1.4 getaddrinfo raises:

test_forward_proxy.exe: [INFO] GET http://foo.invalid
[exception] Eio.Io Net Address_lookup_failed NONAME (name or service is not known),
  looking up "foo.invalid" (service "http")

[avsm]

lgtm, and backwards compatible with older eio