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: https://github.com/nodejs/node/pull/10171
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

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

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