net/http: TestServerCancelsReadTimeoutWhenIdle flake on linux-amd64-race

[josharian]

Seen on a trybot run:

--- FAIL: TestServerCancelsReadTimeoutWhenIdle (0.32s)
	serve_test.go:5111: Get http://127.0.0.1:38397: http: server closed idle connection
2017/03/19 00:55:14 http: TLS handshake error from 127.0.0.1:58706: read tcp 127.0.0.1:37994->127.0.0.1:58706: use of closed file or network connection
2017/03/19 00:55:14 http2: server: error reading preface from client 127.0.0.1:51512: read tcp 127.0.0.1:45423->127.0.0.1:51512: use of closed file or network connection
FAIL
FAIL	net/http	10.648s

https://storage.googleapis.com/go-build-log/901859c0/linux-amd64-race_be98d0fb.log

[bradfitz]

And a different variant:
https://storage.googleapis.com/go-build-log/9b8906b4/linux-amd64-race_05e3a2f0.log

[bradfitz]

Another:
https://storage.googleapis.com/go-build-log/baa28ca2/linux-amd64-race_6ec8ee17.log