go.crypto/ssh/agent: add key type to testAgentInterface failure messages.
go.crypto/ssh/agent: add key type to testAgentInterface failure messages.

R=agl, dave, jpsugar
CC=golang-codereviews
https://golang.org/cl/88260043

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