test: refactor test-stream2-writable

* Use common.mustCall() to confirm that _write() is called only once.
* Check that _write() is called with the correct argument

PR-URL: nodejs#13823
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

test/parallel/test-stream2-writable.js

@@ -294,7 +294,9 @@ for (let i = 0; i < chunks.length; i++) {
 {
   // Verify that end(chunk) twice is an error
   const w = new W();
-  w._write = common.noop;
+  w._write = common.mustCall((msg) => {
+    assert.strictEqual(msg.toString(), 'this is the end');
+  });
   let gotError = false;
   w.on('error', function(er) {
     gotError = true;