client(v2)/integration: test fails with Go 1.8

[gyuho]

=== RUN   TestV2NoRetryEOF
--- FAIL: TestV2NoRetryEOF (0.00s)
	client_test.go:60: #0: expected 1 try, got 0
	client_test.go:60: #1: expected 1 try, got 0
=== RUN   TestV2NoRetryNoLeader
--- FAIL: TestV2NoRetryNoLeader (0.00s)
	client_test.go:84: #0: expected "no leader", got invalid URL port "123.22696.sock"
	client_test.go:87: #0: expected 1 request, got 0
	client_test.go:84: #1: expected "no leader", got invalid URL port "123.22696.sock"
	client_test.go:87: #1: expected 1 request, got 0

--- FAIL: TestV2NoRetryEOF (0.00s)
	client_test.go:60: #0: expected 1 try, got 0
	client_test.go:60: #1: expected 1 try, got 0
=== RUN   TestV2NoRetryNoLeader
==================
WARNING: DATA RACE
Write at 0x00c42001f4dc by goroutine 63:
  internal/race.Write()
      /usr/local/golang/1.8/go/src/internal/race/race.go:41 +0x38
  sync.(*WaitGroup).Wait()
      /usr/local/golang/1.8/go/src/sync/waitgroup.go:129 +0x14b
  net/http/httptest.(*Server).Close()
      /usr/local/golang/1.8/go/src/net/http/httptest/server.go:189 +0x2e7
  github.com/coreos/etcd/client/integration.TestV2NoRetryNoLeader()
      /home/runner/workspace/src/github.com/coreos/etcd/gopath/src/github.com/coreos/etcd/client/integration/client_test.go:90 +0x74b
  testing.tRunner()
      /usr/local/golang/1.8/go/src/testing/testing.go:657 +0x107

Previous read at 0x00c42001f4dc by goroutine 64:
  internal/race.Read()
      /usr/local/golang/1.8/go/src/internal/race/race.go:37 +0x38
  sync.(*WaitGroup).Add()
      /usr/local/golang/1.8/go/src/sync/waitgroup.go:71 +0x26b
  net/http/httptest.(*Server).goServe()
      /usr/local/golang/1.8/go/src/net/http/httptest/server.go:232 +0x47
  net/http/httptest.(*Server).Start()
      /usr/local/golang/1.8/go/src/net/http/httptest/server.go:98 +0x142

Goroutine 63 (running) created at:
  testing.(*T).Run()
      /usr/local/golang/1.8/go/src/testing/testing.go:697 +0x543
  testing.runTests.func1()
      /usr/local/golang/1.8/go/src/testing/testing.go:882 +0xaa
  testing.tRunner()
      /usr/local/golang/1.8/go/src/testing/testing.go:657 +0x107
  testing.runTests()
      /usr/local/golang/1.8/go/src/testing/testing.go:888 +0x4e0
  testing.(*M).Run()
      /usr/local/golang/1.8/go/src/testing/testing.go:822 +0x1c3
  github.com/coreos/etcd/client/integration.TestMain()
      /home/runner/workspace/src/github.com/coreos/etcd/gopath/src/github.com/coreos/etcd/client/integration/main_test.go:15 +0x38
  main.main()
      github.com/coreos/etcd/client/integration/_test/_testmain.go:44 +0x20f

Goroutine 64 (finished) created at:
  github.com/coreos/etcd/client/integration.TestV2NoRetryNoLeader()
      /home/runner/workspace/src/github.com/coreos/etcd/gopath/src/github.com/coreos/etcd/client/integration/client_test.go:75 +0x2d0
  testing.tRunner()
      /usr/local/golang/1.8/go/src/testing/testing.go:657 +0x107
==================
--- FAIL: TestV2NoRetryNoLeader (0.01s)
	client_test.go:84: #0: expected "no leader", got invalid URL port "123.22696.sock"
	client_test.go:87: #0: expected 1 request, got 0
	client_test.go:84: #1: expected "no leader", got invalid URL port "123.22696.sock"
	client_test.go:87: #1: expected 1 request, got 0
	testing.go:610: race detected during execution of test

https://semaphoreci.com/coreos/etcd/branches/master/builds/1684