test: match gccgo error messages for bug345

bug345.dir/main.go:25:18: error: argument 1 has incompatible type (need explicit conversion; missing method ‘Write’)
bug345.dir/main.go:27:8: error: argument 1 has incompatible type

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5556043