Add false positive checks to unicode test

tests/compile-fail/unicode.rs

@@ -5,16 +5,19 @@
 fn zero() {
     print!("Here >​< is a ZWS, and ​another");
                //~^ ERROR zero-width space detected
+    print!("This\u{200B}is\u{200B}fine");
 }
 
 #[deny(unicode_not_nfc)]
 fn canon() {
     print!("̀àh?"); //~ERROR non-nfc unicode sequence detected
+    print!("a\u{0300}h?"); // also okay
 }
 
 #[deny(non_ascii_literal)]
 fn uni() {
     print!("Üben!"); //~ERROR literal non-ASCII character detected
+    print!("\u{DC}ben!"); // this is okay 
 }
 
 fn main() {