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

src/cmd/go/test.go

@@ -228,7 +228,7 @@ func runTest(cmd *Command, args []string) {
 	for _, p := range pkgs {
 		buildTest, runTest, err := b.test(p)
 		if err != nil {
-			errorf("%s: %s", p, err)
+			errorf("%s", err)
 			continue
 		}
 		builds = append(builds, buildTest)