Update tests

Author: camelid

src/test/ui/parser/keyword-const-as-identifier.rs

@@ -1,3 +1,5 @@
 fn main() {
-    let const = "foo"; //~ ERROR expected `{`, found `=`
+    let const = "foo";
+    //~^ ERROR expected `{`, found `=`
+    //~| ERROR inline-const is experimental [E0658]
 }

src/test/ui/parser/keyword-const-as-identifier.stderr

@@ -4,5 +4,15 @@ error: expected `{`, found `=`
 LL |     let const = "foo";
    |               ^ expected `{`
 
-error: aborting due to previous error
+error[E0658]: inline-const is experimental
+  --> $DIR/keyword-const-as-identifier.rs:2:9
+   |
+LL |     let const = "foo";
+   |         ^^^^^
+   |
+   = note: see issue #76001 <https://github.com/rust-lang/rust/issues/76001> for more information
+   = help: add `#![feature(inline_const)]` to the crate attributes to enable
+
+error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0658`.