x/net/http2: failures with "client: missing connection" in TestTransportBodyReadError_*

[bcmills]

--- FAIL: TestTransportBodyReadError_Immediately (0.00s)
    transport_test.go:4590: server: ReadFrame = [FrameHeader WINDOW_UPDATE len=4], <nil>
    transport_test.go:774: client: missing connection
    transport_test.go:4590: server: ReadFrame = [FrameHeader HEADERS flags=END_HEADERS stream=1 len=36], <nil>
    transport_test.go:4590: server: ReadFrame = <nil>, read tcp4 127.0.0.1:51605->127.0.0.1:39284: use of closed network connection
FAIL
exitcode=1FAIL	golang.org/x/net/http2	22.996s

2021-01-19T19:43:25-5f4716e/android-amd64-emu
2020-10-02T20:24:02-0a1ea39/android-amd64-emu
2020-05-20T00:47:42-59133d7/android-amd64-emu
2019-12-04T02:50:24-5ee1b9f/aix-ppc64
2019-10-14T21:28:45-da9a3fd/android-amd64-emu
2019-09-26T02:58:31-c00fd9a/android-386-emu
2019-09-09T00:30:24-a7b1673/linux-ppc64-buildlet
2019-08-13T14:13:03-74dc4d7/dragonfly-amd64
2019-07-24T01:30:45-ca1201d/android-386-emu
2019-04-24T11:20:56-4829fb1/linux-arm64-packet
2019-04-03T14:48:56-b630fd6/android-386-emu
2019-03-26T08:36:53-a33f666/linux-amd64-race
2018-11-14T22:03:01-adae6a3/freebsd-arm-paulzhol

CC @bradfitz @tombergan @empijei @neild

[networkimprov]

cc @fraenkel