lib-http: client: Fixed request-specific attempt timeout.

lib-http: client: Fixed request-specific attempt timeout.

This is the timeout applied to a single request attempt. Using http_client_request_set_attempt_timeout_msecs() this can be set for a specific request.
However, this was mostly ignored for requests that weren't in the process of handling response payload.
Instead, the global request_timeout_msecs client setting was used.

Also amended the (currently manual) test suite with tests that demonstated the problem and now verify the fix.