add system.du

src/test/system-test.js

@@ -36,5 +36,12 @@ describe('System', function () {
         expect(memory.memory).to.be.a('number');
         expect(memory.swap).to.be.a('number');
     });
+
+    it('can get du', async function () {
+        const usage = await system.du(__dirname);
+        expect(usage).to.be.a('number');
+        console.log(usage);
+    });
+
 });