net/http: apparent TestServerAllowsBlockingRemoteAddr flakes due to hard-coded timeouts

[bcmills]

2019-12-16T20:38:31-f7f9866/plan9-386-0intro
2019-06-12T14:58:18-65f53da/plan9-amd64-9front

--- FAIL: TestServerAllowsBlockingRemoteAddr (1.08s)
    serve_test.go:1300: Request 1: Get "http://127.0.0.1:55517": context deadline exceeded (Client.Timeout exceeded while awaiting headers)
    serve_test.go:1349: response 1 addr = ""; want "RA:21.21.21.21:21"
http.test 1018696: warning: process exceeds 100 file descriptors
FAIL
FAIL	net/http	114.189s

As far as I can tell, the root cause is the hard-coded time.Second here:

go/src/net/http/serve_test.go

Lines 1326 to 1330 in 931fe39

	select {
	case conn2 = <-conns:
	case <-time.After(time.Second):
	t.Fatal("Second Accept didn't happen")
	}

It's not obvious to me why a timeout is needed there at all. If the test deadlocks, we presumably want a goroutine dump anyway.

CC @bradfitz @0intro @fhs

[bcmills]

That's the regression test for #12943.