test: fix flaky test_cancel_and_errinj

This change tries to fix the following sporadic test error:

    [007] not ok 4	http_client.sock_family:"AF_INET".test_cancel_and_errinj
    [007] #   ...arantool/tarantool/test/app-luatest/http_client_test.lua:221: Timeout check - status
    [007] #   expected: 408, actual: 200

Fixes tarantool/tarantool-qa#154

NO_DOC=testing
NO_TEST=testing
NO_CHANGELOG=testing