crypto/tls: fix flakey test.
crypto/tls: fix flakey test.

A test added in b37d2fdcc4d9 didn't work with some values of GOMAXPROCS
because the defer statements were in the wrong order: the Pipe could be
closed before the TLS Client was.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/9187047

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