Remove external df module

It has some parsing issues with locale

src/test/df-test.js

@@ -0,0 +1,36 @@
+/* jslint node:true */
+/* global it:false */
+/* global describe:false */
+/* global before:false */
+/* global after:false */
+
+'use strict';
+
+const common = require('./common.js'),
+    df = require('../df.js'),
+    expect = require('expect.js');
+
+describe('System', function () {
+    const { setup, cleanup } = common;
+
+    before(setup);
+    after(cleanup);
+
+    it('can get disks', async function () {
+        // does not work on archlinux 8!
+        if (require('child_process').execSync('uname -a').toString().indexOf('-arch') !== -1) return;
+
+        const disks = await df.disks();
+        expect(disks).to.be.ok();
+        expect(disks.some(d => d.mountpoint === '/')).to.be.ok();
+    });
+
+    it('can get file', async function () {
+        // does not work on archlinux 8!
+        if (require('child_process').execSync('uname -a').toString().indexOf('-arch') !== -1) return;
+
+        const disks = await df.file(__dirname);
+        expect(disks).to.be.ok();
+        expect(disks.mountpoint).to.be('/home');
+    });
+});