Move libXtest into libX/tests

This change moves:

1. `libcoretest` into `libcore/tests`
2. `libcollectionstest` into `libcollections/tests`

This is a follow-up to #39561.

src/libcore/tests/array.rs

@@ -0,0 +1,28 @@
+// Copyright 2014 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.
+use core::array::FixedSizeArray;
+
+#[test]
+fn fixed_size_array() {
+    let mut array = [0; 64];
+    let mut zero_sized = [(); 64];
+    let mut empty_array = [0; 0];
+    let mut empty_zero_sized = [(); 0];
+
+    assert_eq!(FixedSizeArray::as_slice(&array).len(), 64);
+    assert_eq!(FixedSizeArray::as_slice(&zero_sized).len(), 64);
+    assert_eq!(FixedSizeArray::as_slice(&empty_array).len(), 0);
+    assert_eq!(FixedSizeArray::as_slice(&empty_zero_sized).len(), 0);
+
+    assert_eq!(FixedSizeArray::as_mut_slice(&mut array).len(), 64);
+    assert_eq!(FixedSizeArray::as_mut_slice(&mut zero_sized).len(), 64);
+    assert_eq!(FixedSizeArray::as_mut_slice(&mut empty_array).len(), 0);
+    assert_eq!(FixedSizeArray::as_mut_slice(&mut empty_zero_sized).len(), 0);
+}