test: fix lint error

Author: DevPres

test/parallel/test-stream-pipe-unpipe-streams.js

@@ -64,7 +64,7 @@ assert.strictEqual(source._readableState.pipes.length, 0);
         'listener which is `unpipeChecker`'
       );
       dest.removeListener('unpipe', unpipeChecker);
-      for (let eventName of destCheckEventNames) {
+      for (const eventName of destCheckEventNames) {
         assert.strictEqual(
           dest.listenerCount(eventName), 0,
           `destination{${currentDestId}}'s '${eventName}' event ` +