net/smtp: remove data race from TestSendMail.
net/smtp: remove data race from TestSendMail.

A data race was found in TestSendMail by the race detector.

Fixes issue 4559.

R=golang-dev, bradfitz, dave, rsc
CC=golang-dev
https://codereview.appspot.com/6944057

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

dsriram/golang,apparentlymart/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,gnanderson/go,apparentlymart/golang,gnanderson/go,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,apparentlymart/golang,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme