Update const fn tests

src/test/ui/consts/min_const_fn/bad_const_fn_body_ice.stderr

@@ -1,10 +1,12 @@
-error: heap allocations are not allowed in const fn
+error[E0723]: heap allocations are not allowed in const fn (see issue #57563)
   --> $DIR/bad_const_fn_body_ice.rs:2:5
    |
 LL |     vec![1, 2, 3] //~ ERROR heap allocations are not allowed in const fn
    |     ^^^^^^^^^^^^^
    |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0723`.

src/test/ui/consts/min_const_fn/cast_errors.stderr

@@ -1,32 +1,43 @@
-error: unsizing casts are not allowed in const fn
+error[E0723]: unsizing casts are not allowed in const fn (see issue #57563)
   --> $DIR/cast_errors.rs:3:41
    |
 LL | const fn unsize(x: &[u8; 3]) -> &[u8] { x }
    |                                         ^
+   |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
 
-error: function pointers in const fn are unstable
+error[E0723]: function pointers in const fn are unstable (see issue #57563)
   --> $DIR/cast_errors.rs:5:23
    |
 LL | const fn closure() -> fn() { || {} }
    |                       ^^^^
+   |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
 
-error: function pointers in const fn are unstable
+error[E0723]: function pointers in const fn are unstable (see issue #57563)
   --> $DIR/cast_errors.rs:8:5
    |
 LL |     (|| {}) as fn();
    |     ^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
 
-error: function pointers in const fn are unstable
+error[E0723]: function pointers in const fn are unstable (see issue #57563)
   --> $DIR/cast_errors.rs:11:28
    |
 LL | const fn reify(f: fn()) -> unsafe fn() { f }
    |                            ^^^^^^^^^^^
+   |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
 
-error: function pointers in const fn are unstable
+error[E0723]: function pointers in const fn are unstable (see issue #57563)
   --> $DIR/cast_errors.rs:13:21
    |
 LL | const fn reify2() { main as unsafe fn(); }
    |                     ^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
 
 error: aborting due to 5 previous errors
 
+For more information about this error, try `rustc --explain E0723`.

src/test/ui/consts/min_const_fn/cmp_fn_pointers.stderr

@@ -1,8 +1,11 @@
-error: function pointers in const fn are unstable
+error[E0723]: function pointers in const fn are unstable (see issue #57563)
   --> $DIR/cmp_fn_pointers.rs:1:14
    |
 LL | const fn cmp(x: fn(), y: fn()) -> bool { //~ ERROR function pointers in const fn are unstable
    |              ^
+   |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0723`.

src/test/ui/consts/min_const_fn/loop_ice.stderr

@@ -1,8 +1,11 @@
-error: loops are not allowed in const fn
+error[E0723]: loops are not allowed in const fn (see issue #57563)
   --> $DIR/loop_ice.rs:2:5
    |
 LL |     loop {} //~ ERROR loops are not allowed in const fn
    |     ^^^^^^^
+   |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0723`.