crypto/tls: darwin race builders failing on BenchmarkHandshakeServer/ECDHE-X25519-ECDSA-P256

[thanm]

The darwin race builder has been failing for some time with errors or this form:

=== RUN   BenchmarkHandshakeServer/ECDHE-P256-RSA
BenchmarkHandshakeServer/ECDHE-P256-RSA
    handshake_server_test.go:1346: #4: read tcp 127.0.0.1:62314->127.0.0.1:62310: use of closed network connection
        Read 0, wanted 51, got , wanted 140303000101160303002800000000000000005df832a2c7d4ac17570d9fc1962a774871f425eccda764cb20ea6acc31f0cb8b
    handshake_server_test.go:1349: #4: mismatch on read: got:000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 want:140303000101160303002800000000000000005df832a2c7d4ac17570d9fc1962a774871f425eccda764cb20ea6acc31f0cb8b

Representative logs:

https://build.golang.org/log/a53a964e3bc25c4d3b3e4883e320e9f8b060f41b
https://ci.chromium.org/ui/p/golang/builders/ci/gotip-darwin-amd64-race/b8746453827322458849/overview

I can reproduce on a gomote, e.g.

$ export MOTE=thanm-gotip-darwin-amd64-race-0
$  gomote run $MOTE go/bin/go test -count=1 -bench=BenchmarkHandshakeServer crypto/tls

It looks as though this started at CL 586655 from inspecting the builder logs.

CC @FiloSottile @rolandshoemaker