add createReadStream test

src/__tests__/volume.test.ts

@@ -1444,4 +1444,29 @@ describe('volume', () => {
       });
     });
   });
+  describe('.createReadStream', () => {
+    it('accepts filehandle as fd option', done => {
+      const vol = Volume.fromJSON({
+        '/test.txt': 'Hello',
+      });
+      vol.promises.open('/test.txt', 'r').then(fh => {
+        const readStream = vol.createReadStream(
+          '/this/should/be/ignored',
+          { fd: fh },
+        );
+        readStream.setEncoding('utf8');
+        let readData = '';
+        readStream.on('readable', () => {
+          const chunk = readStream.read();
+          if (chunk != null) readData += chunk;
+        });
+        readStream.on('end', () => {
+          expect(readData).toEqual('Hello');
+          done();
+        });
+      }).catch((err) => {
+        expect(err).toBeNull();
+      });
+    });
+  });
 });