Move f64::NAN ui tests into library

Author: chansuke

library/core/tests/num/nan.rs

@@ -0,0 +1,8 @@
+#[test]
+fn test_nan() {
+  use core::f64;
+  let x = "NaN".to_string();
+  assert_eq!(format!("{}", f64::NAN), x);
+  assert_eq!(format!("{:e}", f64::NAN), x);
+  assert_eq!(format!("{:E}", f64::NAN), x);
+}