Document that gopherjs test requires system calls Node module (or make it not require it, if possible).

> I'm almost certain `gopherjs test` worked previously without installing the syscall module (I have never installed it in the past AFAIK). So probably a regression, would be my bet.

See https://github.com/gopherjs/gopherjs/issues/624#issuecomment-294386098 for details.

**Edit:** Also see #815 for a minimal reproduce case.