adjust error messages in tests

Author: RalfJung

tests/compile-fail/atomic_non_integer_arithmetic.rs

@@ -10,6 +10,6 @@ pub fn main() {
     unsafe {
         use crate::rusti::*;
 
-        ctlz_nonzero(0u8); //~ ERROR ctlz_nonzero called on 0
+        ctlz_nonzero(0u8); //~ ERROR `ctlz_nonzero` called on 0
     }
 }

tests/compile-fail/ctlz_nonzero.rs

@@ -10,6 +10,6 @@ pub fn main() {
     unsafe {
         use crate::rusti::*;
 
-        cttz_nonzero(0u8); //~ ERROR cttz_nonzero called on 0
+        cttz_nonzero(0u8); //~ ERROR `cttz_nonzero` called on 0
     }
 }

tests/compile-fail/cttz_nonzero.rs

@@ -2,7 +2,7 @@
 
 use std::intrinsics::*;
 
-//error-pattern: Overflowing shift by 64 in unchecked_shr
+//error-pattern: Overflowing shift by 64 in `unchecked_shr`
 
 fn main() {
     unsafe {