Added tests for writing to static mut's in statics.

Author: Alexander Regueiro

src/test/compile-fail/write-to-static-mut-in-static.rs

@@ -0,0 +1,22 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(const_let)]
+
+pub static mut A: u32 = 0;
+pub static mut B: () = unsafe { A = 1; };
+//~^ ERROR statements in statics are unstable
+
+pub static mut C: u32 = unsafe { C = 1; 0 };
+//~^ ERROR statements in statics are unstable
+
+pub static D: u32 = D;
+
+fn main() {}