add test for createWriteStream accepting fh as fd

src/__tests__/volume.test.ts

@@ -1,3 +1,4 @@
+import { promisify } from 'util';
 import { URL } from 'url';
 import { Link } from '../node';
 import Stats from '../Stats';
@@ -1431,4 +1432,16 @@ describe('volume', () => {
       expect(new StatWatcher(vol).vol).toBe(vol);
     });
   });
+  describe('.createWriteStream', () => {
+    it('accepts filehandle as fd option', async () => {
+      const vol = new Volume();
+      const fh = await vol.promises.open('/test.txt', 'wx', 0o600);
+      const writeStream = vol.createWriteStream('', { fd: fh });
+      await promisify(writeStream.write.bind(writeStream))(Buffer.from('Hello'));
+      await promisify(writeStream.close.bind(writeStream))();
+      expect(vol.toJSON()).toEqual({
+        '/test.txt': 'Hello',
+      });
+    });
+  });
 });