fix existing tests

fix thread-local example to no longer write to pointers derived from a shared ref;
fix compile-fail test

Author: RalfJung

tests/compile-fail/modifying_constants.rs

@@ -1,3 +1,5 @@
+// This should fail even without validation
+// compile-flags: -Zmiri-disable-validation
 
 fn main() {
     let x = &1; // the `&1` is promoted to a constant, but it used to be that only the pointer is marked static, not the pointee

tests/run-pass/thread-local.rs

@@ -56,7 +56,7 @@ fn main() {
         create(None); // check that the no-dtor case works
 
         // Initialize the keys we use to check destructor ordering
-        for (key, global) in KEYS.iter_mut().zip(GLOBALS.iter()) {
+        for (key, global) in KEYS.iter_mut().zip(GLOBALS.iter_mut()) {
             *key = create(Some(mem::transmute(dtor as unsafe extern fn(*mut u64))));
             set(*key, global as *const _ as *mut _);
         }