Fix a typo in a rt::io::signal test

It was pretty much a miracle that these tests were ever passing. They would
never have passed in the single threaded case because only one sigint in the
tests is ever generated, but when run in parallel two sigints will be generated.

src/libstd/rt/io/signal.rs

@@ -183,7 +183,7 @@ mod test {
             Interrupt => (),
             s => fail!("Expected Interrupt, got {:?}", s),
         }
-        match s1.port.recv() {
+        match s2.port.recv() {
             Interrupt => (),
             s => fail!("Expected Interrupt, got {:?}", s),
         }