system: add tests for fs usage route

src/docker.js

@@ -679,8 +679,10 @@ async function info() {
     return result;
 }
 
-async function df() {
-    const [error, result] = await safe(gConnection.df());
+async function df(options) {
+    assert.strictEqual(typeof options, 'object');
+
+    const [error, result] = await safe(gConnection.df(options));
     if (error) throw new BoxError(BoxError.DOCKER_ERROR, `Error connecting to docker: ${error.message}`);
     return result;
 }