Tests: masked nginx bug in proxy next upstream. When iterating through several next upstreams per a worker cycle, a previously reported event about upstream connection error may be improperly applied to the next upstream, thus leading to the invalid connection error. E.g., in kqueue, where the problem is visible, this is caused by "ev->instance" that does not tolerate more than one next upstream at once, and kevents placed on the kqueue separately for read and write events. The change is to limit test case to the only one next upstream.