Skip print-stdout-eprint-stderr test on emscripten

Author: zackw

src/test/run-pass/print-stdout-eprint-stderr.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// ignore-emscripten spawning processes is not supported
+
 use std::{env, process};
 
 fn child() {