test: change var to let

PR-URL: #30444
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/message/stdin_messages.js

@@ -40,8 +40,8 @@ const queue =
   [ 'with(this){__filename}',
     '42',
     'throw new Error("hello")',
-    'var x = 100; y = x;',
-    'var ______________________________________________; throw 10' ];
+    'let x = 100; y = x;',
+    'let ______________________________________________; throw 10' ];
 
 function go() {
   const c = queue.shift();

test/message/stdin_messages.out

@@ -48,7 +48,7 @@ Error: hello
     at endReadableNT (_stream_readable.js:*:*)
 100
 [stdin]:1
-var x = 100; y = x;
+let x = 100; y = x;
                ^
 
 ReferenceError: y is not defined
@@ -64,13 +64,13 @@ ReferenceError: y is not defined
     at endReadableNT (_stream_readable.js:*:*)
 
 [stdin]:1
-var ______________________________________________; throw 10
+let ______________________________________________; throw 10
                                                     ^
 10
 (Use `node --trace-uncaught ...` to show where the exception was thrown)
 
 [stdin]:1
-var ______________________________________________; throw 10
+let ______________________________________________; throw 10
                                                     ^
 10
 (Use `node --trace-uncaught ...` to show where the exception was thrown)