ssh: fix flaky TestInvalidServerMessage
ssh: fix flaky TestInvalidServerMessage

When shutting down the test, we sometimes see EOF on the server's side
of the connection and sometimes ECONNRESET. In the latter case, based
on timing, it was possible that the server loop would hit Errorf during
shutdown and cause the test to fail.

R=dave
CC=golang-dev
http://codereview.appspot.com/6125047

cmars/go.crypto,isislovecruft/go.crypto,drhax9908/go.crypto,tonnerre/golang-go.crypto,dizzyd/go.crypto,cmars/sshlurp-go.crypto,xman1101/terminal,drhax9908/go.crypto,polydawn-ports/go.crypto,tonnerre/golang-go.crypto,dizzyd/go.crypto,cmars/go.crypto,polydawn-ports/go.crypto,cmars/sshlurp-go.crypto,xman1101/terminal,isislovecruft/go.crypto