XFAIL sio-ctx

Reportedly fails sometimes, probably when RUST_THREADS > 1

Author: brson

src/test/run-pass/sio-ctx.rs

@@ -1,3 +1,8 @@
+// Fails periodically, probably with RUST_THREADS > 1
+// xfail-stage1
+// xfail-stage2
+// xfail-stage3
+
 use std;
 import std::sio;