test: increase test coverage of BufferList

Add tests for edges cases of BufferList:
- test operations when the length is 0
- test operations when the list only has one element
PR-URL: #10171
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: joyeecheung

test/parallel/test-stream-buffer-list.js

@@ -0,0 +1,27 @@
+// Flags: --expose_internals
+'use strict';
+require('../common');
+constassert=require('assert');
+constBufferList=require('internal/streams/BufferList');
+
+// Test empty buffer list.
+constemptyList=newBufferList();
+
+emptyList.shift();
+assert.deepStrictEqual(emptyList,newBufferList());
+
+assert.strictEqual(emptyList.join(','),'');
+
+assert.deepStrictEqual(emptyList.concat(0),Buffer.alloc(0));
+
+// Test buffer list with one element.
+constlist=newBufferList();
+list.push('foo');
+
+assert.strictEqual(list.concat(1),'foo');
+
+assert.strictEqual(list.join(','),'foo');
+
+constshifted=list.shift();
+assert.strictEqual(shifted,'foo');
+assert.deepStrictEqual(list,newBufferList());