test: change var to const in parallel/test-stream-transform-final*

PR-URL: #30448
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: khoumani

Diff for: test/parallel/test-stream-transform-final-sync.js

@@ -7,7 +7,7 @@ let state = 0;
 
 /*
 What you do
-var stream = new stream.Transform({
+const stream = new stream.Transform({
   transform: function transformCallback(chunk, _, next) {
     // part 1
     this.push(chunk);

Diff for: test/parallel/test-stream-transform-final.js

@@ -7,7 +7,7 @@ let state = 0;
 
 /*
 What you do
-var stream = new stream.Transform({
+const stream = new stream.Transform({
   transform: function transformCallback(chunk, _, next) {
     // part 1
     this.push(chunk);