Xfail-fast, because these tests use failure

Author: eholk

src/test/run-pass/issue-3168.rs

@@ -1,3 +1,5 @@
+// xfail-fast
+
 fn main() {
     let (c,p) = pipes::stream();
     do task::try {

src/test/run-pass/issue-3176.rs

@@ -1,3 +1,5 @@
+// xfail-fast
+
 import pipes::{select2, selectable};
 
 fn main() {