Rollup merge of #79227 - sasurau4:test/move-cell-test-to-lib-core, r=jyn514

Remove const_fn_feature_flags test

## Overview

Helps with #76268

I found `const_fn_feature_flags` is targeting feature-gate and remove it.

r? ``@matklad``

src/test/ui/consts/const-fn-feature-flags.rs

@@ -1,13 +0,0 @@
-// run-pass
-// Test use of stabilized const fns in std formerly using individual feature gates.
-
-use std::cell::Cell;
-
-const CELL: Cell<i32> = Cell::new(42);
-
-fn main() {
-    let v = CELL.get();
-    CELL.set(v+1);
-
-    assert_eq!(CELL.get(), v);
-}