ssh/test: deflake session test.
ssh/test: deflake session test.

The session test previously had a one second timeout for the output of
stty and this was leading to flakiness. This change removes the timeout
since go test has a generic timeout mechanism.

Additionally, the test was looking for "-echo" in the output to test
the value of the echo flag. However, there are also typically "echoe",
"echok" and "echonl" flags, and "-echo" could be a prefix of any of
time. Thus we now also match a trailing space.

R=golang-dev, rsc, extraterrestrial.neighbour
CC=golang-dev
https://golang.org/cl/7579043

wooutl/golangcrypto,juniorz/crypto,conseweb/golangcrypto,golang/crypto,cmars/tools,juniorz/crypto,wooutl/golangcrypto,mattharden/crypto,tca/crypto,kim-racktop/crypto,guaiguaihw/crypto,a-palchikov/crypto,zj8487/crypto,bford/golang-x-crypto,pki-io/crypto,Carrotman42/crypto,tca/crypto,pki-io/crypto,ScriptRock/crypto,bford/golang-x-crypto,cmars/tools,zj8487/crypto,lucevers/crypto,a-palchikov/crypto,golang/crypto,thomaso-mirodin/crypto,conseweb/golangcrypto,cheffo/crypto,cmars/tools,golang/crypto,thomaso-mirodin/crypto,kim-racktop/crypto,Carrotman42/crypto,progrium/crypto,cmars/tools,mattharden/crypto,progrium/crypto,CodyGuo/crypto,CodyGuo/crypto,guaiguaihw/crypto,ScriptRock/crypto,lucevers/crypto,cheffo/crypto