cmd/dist: add command output for dist tests

It is very useful to see which test commands are executed.
This is of global use, but I wrote it for #11654.

Change-Id: I9bfc8e55d5bef21f4c49b917f58bc9a44aefcade
Reviewed-on: https://go-review.googlesource.com/12510Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/dist/test.go

@@ -519,6 +519,9 @@ func (t *tester) dirCmd(dir string, bin string, args ...string) *exec.Cmd {
 	}
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
+	if vflag > 1 {
+		errprintf("%s\n", strings.Join(cmd.Args, " "))
+	}
 	return cmd
 }