test: ensure sandbox dir is present

Fix test/browser.test.js failing on CI due to sandbox dir not present
on the first run.

test/browser.test.js

@@ -9,6 +9,8 @@ var vm = require('vm');
 describe('browser support', function() {
   this.timeout(60000); // 60s to give browserify enough time to finish
 
+  beforeEach(sandbox.reset);
+
   it('has API for bundling and executing boot instructions', function(done) {
     var appDir = path.resolve(__dirname, './fixtures/browser-app');