go test: don't try to print package with error message.

go test: don't try to print package with error message.
If there's a error compiling a _test.go file, the error
message tries to print a *Package with %s. There's no String
method on *Package, so the error message looks bad.
Since the error messages identify the file in question
anyway, this CL removes the package from the error message.

R=rsc, gri
CC=golang-dev
https://golang.org/cl/5520045