organize compile-fail tests in folders

tests/compile-fail/function_pointers/execute_memory.rs

@@ -0,0 +1,12 @@
+// Validation makes this fail in the wrong place
+// compile-flags: -Zmiri-disable-validation
+
+#![feature(box_syntax)]
+
+fn main() {
+    let x = box 42;
+    unsafe {
+        let f = std::mem::transmute::<Box<i32>, fn()>(x);
+        f() //~ ERROR function pointer but it does not point to a function
+    }
+}