I've packed testscript into Homebrew, so now you can do this.

brew install testscript

The testscript, if executed without parameters, waits for input from stdin, while --help says

The special filename "-" is interpreted as the standard input.

Because there is already "-" for handling stdin input, I think that program without parameters should output --help. However,

I am not sure where to report the bug/request this feature?

Should it be here, or somewhere in Go internals tracker? How does this code gets changes from upstream?