exp/ssh: cleanup client auth tests

exp/ssh: cleanup client auth tests

This CL cleans up the client auth tests, making the
individual test body more manageable.

Also, adds tests for rsa and dsa key negotiation.

Finally, remove the package level use of the variable
strings, which avoids conflicting with the strings pkg.

R=gustav.paul, agl, n13m3y3r, rsc
CC=golang-dev
https://golang.org/cl/5447049