Add test to check stdout flushing during shutdown.

src/test/ui/stdout-during-shutdown.rs

@@ -0,0 +1,14 @@
+// run-pass
+// check-run-results
+
+#![feature(rustc_private)]
+
+extern crate libc;
+
+fn main() {
+    extern "C" fn bye() {
+        print!(", world!");
+    }
+    unsafe { libc::atexit(bye) };
+    print!("hello");
+}

src/test/ui/stdout-during-shutdown.run.stdout

@@ -0,0 +1 @@
+hello, world!