Move and rewrite tests to use &mut in constants

Author: pvdrz

src/test/ui/consts/const-mut-refs/const_mut_refs.rs

@@ -0,0 +1,15 @@
+// run-pass
+
+#![feature(const_mut_refs)]
+
+struct Foo {
+    x: usize
+}
+
+const fn bar(foo: &mut Foo) -> usize {
+    foo.x + 1
+}
+
+fn main() {
+    let _: [(); bar(&mut Foo { x: 0 })] = [(); 1];
+}

src/test/ui/consts/const-eval/feature-gate-const_fn_mut_refs.rs renamed to src/test/ui/consts/const-mut-refs/feature-gate-const_mut_refs.rs

@@ -1,5 +1,5 @@
 error[E0723]: mutable references in const fn are unstable
-  --> $DIR/feature-gate-const_fn_mut_refs.rs:5:14
+  --> $DIR/feature-gate-const_mut_refs.rs:5:14
    |
 LL | const fn foo(x: &mut i32) -> i32 {
    |              ^