Move various ui const tests to library

Move:
 - `src\test\ui\consts\const-nonzero.rs` to `library\core`
 - `src\test\ui\consts\ascii.rs` to `library\core`
 - `src\test\ui\consts\cow-is-borrowed` to `library\alloc`

Part of rust-lang#76268

library/alloc/tests/borrow.rs

@@ -45,3 +45,16 @@ fn test_from_cow_path() {
     let path = Path::new("hello");
     test_from_cow!(path: &Path);
 }
+
+#[test]
+fn cow_const() {
+    // test that the methods of `Cow` are usable in a const context
+
+    const COW: Cow<'_, str> = Cow::Borrowed("moo");
+
+    const IS_BORROWED: bool = COW.is_borrowed();
+    assert!(IS_BORROWED);
+
+    const IS_OWNED: bool = COW.is_owned();
+    assert!(!IS_OWNED);
+}

library/alloc/tests/lib.rs

@@ -1,5 +1,6 @@
 #![feature(allocator_api)]
 #![feature(box_syntax)]
+#![feature(cow_is_borrowed)]
 #![feature(drain_filter)]
 #![feature(exact_size_is_empty)]
 #![feature(new_uninit)]

library/core/tests/ascii.rs

@@ -397,3 +397,14 @@ fn test_is_ascii_align_size_thoroughly() {
         }
     }
 }
+
+#[test]
+fn ascii_const() {
+    // test that the `is_ascii` methods of `char` and `u8` are usable in a const context
+
+    const CHAR_IS_ASCII: bool = 'a'.is_ascii();
+    assert!(CHAR_IS_ASCII);
+
+    const BYTE_IS_ASCII: bool = 97u8.is_ascii();
+    assert!(BYTE_IS_ASCII);
+}

library/core/tests/nonzero.rs

@@ -195,3 +195,20 @@ fn test_nonzero_from_int_on_err() {
     assert!(NonZeroI8::try_from(0).is_err());
     assert!(NonZeroI32::try_from(0).is_err());
 }
+
+#[test]
+fn nonzero_const() {
+    // test that the methods of `NonZeroX>` are usable in a const context
+    // Note: only tests NonZero8
+
+    const NONZERO: NonZeroU8 = unsafe { NonZeroU8::new_unchecked(5) };
+
+    const GET: u8 = NONZERO.get();
+    assert_eq!(GET, 5);
+
+    const ZERO: Option<NonZeroU8> = NonZeroU8::new(0);
+    assert!(ZERO.is_none());
+
+    const ONE: Option<NonZeroU8> = NonZeroU8::new(1);
+    assert!(ONE.is_some());
+}