net/http: TestTransportEventTrace_h2 flake

[bradfitz]

Just noticed this flake:

https://build.golang.org/log/25f4c603239103f3ffa549abbf917a7e5f4b5be9

--- FAIL: TestTransportEventTrace_h2 (0.01s)
    transport_test.go:3329: expected substring "WroteRequest: {Err:<nil>}" exactly once in output.
    transport_test.go:3355: Output:
        Getting conn for dns-is-faked.golang:40463 ...
        DNS start: {Host:dns-is-faked.golang}
        DNS done: {Addrs:[{IP:127.0.0.1 Zone:}] Err:<nil> Coalesced:false}
        ConnectStart: Connecting to tcp 127.0.0.1:40463 ...
        ConnectDone: connected to tcp 127.0.0.1:40463 = <nil>
        got conn: {Conn:0xc420146000 Reused:false WasIdle:false IdleTime:0s}
        Wait100Continue
        first response byte
        Got100Continue
        got roundtrip.response
        consumed body
FAIL

[bradfitz]

Again in https://go-review.googlesource.com/#/c/28076/2

[bradfitz]

... but the last one was a race too:
https://storage.googleapis.com/go-build-log/bf036893/linux-amd64-race_ce7be7b1.log

[gopherbot]

CL https://golang.org/cl/28078 mentions this issue.