Remove httpbin.org from tests

[cuonglm]

Current tests still use httpbin.org, make the test flaky sometimes.

> for _ in {1..10}; do go1.12.12 test -race -run=TestResponseTimingsWhenTimeout; done
WARN[0007] Request Failed                                error="Get http://httpbin.org/delay/10: net/http: request canceled (Client.Timeout exceeded while awaiting headers)"
PASS
ok      github.com/loadimpact/k6/js/modules/k6/http    8.901s
WARN[0007] Request Failed                                error="Get http://httpbin.org/delay/10: net/http: request canceled (Client.Timeout exceeded while awaiting headers)"
PASS
ok      github.com/loadimpact/k6/js/modules/k6/http    8.840s
WARN[0007] Request Failed                                error="Get http://httpbin.org/delay/10: net/http: request canceled (Client.Timeout exceeded while awaiting headers)"
PASS
ok      github.com/loadimpact/k6/js/modules/k6/http    8.530s
WARN[0007] Request Failed                                error="Get http://httpbin.org/delay/10: net/http: request canceled (Client.Timeout exceeded while awaiting headers)"
PASS
ok      github.com/loadimpact/k6/js/modules/k6/http    8.621s
WARN[0007] Request Failed                                error="Get http://httpbin.org/delay/10: net/http: request canceled (Client.Timeout exceeded while awaiting headers)"
PASS
ok      github.com/loadimpact/k6/js/modules/k6/http    8.874s
WARN[0007] Request Failed                                error="Get http://httpbin.org/delay/10: net/http: request canceled (Client.Timeout exceeded while awaiting headers)"
PASS
ok      github.com/loadimpact/k6/js/modules/k6/http    8.605s
WARN[0007] Request Failed                                error="Get http://httpbin.org/delay/10: net/http: request canceled (Client.Timeout exceeded while awaiting headers)"
--- FAIL: TestResponseTimingsWhenTimeout (7.18s)
        Error Trace:    request_test.go:1776
        Error:          Received unexpected error:
                        Error: expected waiting time to be over 2000ms but was 1068.554 at <eval>:5:8(25)
        Test:           TestResponseTimingsWhenTimeout
FAIL
exit status 1
FAIL    github.com/loadimpact/k6/js/modules/k6/http    7.510s
WARN[0007] Request Failed                                error="Get http://httpbin.org/delay/10: net/http: request canceled (Client.Timeout exceeded while awaiting headers)"
PASS
ok      github.com/loadimpact/k6/js/modules/k6/http    8.947s
WARN[0007] Request Failed                                error="Get http://httpbin.org/delay/10: net/http: request canceled (Client.Timeout exceeded while awaiting headers)"
PASS
ok      github.com/loadimpact/k6/js/modules/k6/http    8.613s
WARN[0007] Request Failed                                error="Get http://httpbin.org/delay/10: net/http: request canceled (Client.Timeout exceeded while awaiting headers)"
PASS
ok      github.com/loadimpact/k6/js/modules/k6/http    8.641s

[na--]

Oops 😅 Sorry, I was the one who removed the test dependencies on the external httpbin.org in #555, but it seems I slipped up and inadvertently added a new one in #1047 😊