test: improve test-stream2-large-read-stall

* use const instead of var
* use assert.strictEqual instead of assert.equal
* use  common.mustCall instead of process.on( 'exit', fn )

PR-URL: https://github.com/nodejs/node/pull/10725
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>

test/parallel/test-stream2-large-read-stall.js

@@ -32,7 +32,9 @@ r.on('readable', function() {
                 rs.length);
 });
 
-r.on('end', common.mustCall(function() {}));
+r.on('end', common.mustCall(function() {
+  assert.strictEqual(pushes, PUSHCOUNT + 1);
+}));
 
 var pushes = 0;
 function push() {
@@ -48,7 +50,3 @@ function push() {
   if (r.push(Buffer.allocUnsafe(PUSHSIZE)))
     setTimeout(push, 1);
 }
-
-process.on('exit', function() {
-  assert.equal(pushes, PUSHCOUNT + 1);
-});