go.crypto/ssh: skip tests that start ssh-agent.
go.crypto/ssh: skip tests that start ssh-agent.

Also stops leaking /tmp/ directories.

Fixes issue 8489.

LGTM=dave, minux
R=dave, minux, agl
CC=golang-codereviews
https://codereview.appspot.com/124010043

Committer: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>

drhax9908/go.crypto,xman1101/terminal,drhax9908/go.crypto,xman1101/terminal