test: writable stream needDrain state

Add a test for _writableState.needDrain.
PR-URL: #8799
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Related: #8686

Author: Italo A. Casas

test/parallel/test-stream-writable-needdrain-state.js

@@ -0,0 +1,23 @@
+'use strict';
+
+constcommon=require('../common');
+conststream=require('stream');
+constassert=require('assert');
+
+consttransform=newstream.Transform({
+transform: _transform,
+highWaterMark: 1
+});
+
+function_transform(chunk,encoding,cb){
+assert.strictEqual(transform._writableState.needDrain,true);
+cb();
+}
+
+assert.strictEqual(transform._writableState.needDrain,false);
+
+transform.write('asdasd',common.mustCall(()=>{
+assert.strictEqual(transform._writableState.needDrain,false);
+}));
+
+assert.strictEqual(transform._writableState.needDrain,true);