cmd/dist: add more logging details when go list std cmd fails

Change-Id: I12e6990b46ea9c733a5718dc5ca67f1fcd2dec66
Reviewed-on: https://go-review.googlesource.com/10754Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/test.go

@@ -274,9 +274,9 @@ func (t *tester) registerTests() {
 	} else {
 		// Use a format string to only list packages and commands that have tests.
 		const format = "{{if (or .TestGoFiles .XTestGoFiles)}}{{.ImportPath}}{{end}}"
-		all, err := exec.Command("go", "list", "-f", format, "std", "cmd").Output()
+		all, err := exec.Command("go", "list", "-f", format, "std", "cmd").CombinedOutput()
 		if err != nil {
-			log.Fatalf("Error running go list std cmd: %v", err)
+			log.Fatalf("Error running go list std cmd: %v, %s", err, all)
 		}
 		for _, pkg := range strings.Fields(string(all)) {
 			t.registerStdTest(pkg)