convert more execSync to async

src/test/shell-test.js

@@ -4,7 +4,8 @@
 
 'use strict';
 
-const expect = require('expect.js'),
+const BoxError = require('../boxerror.js'),
+    expect = require('expect.js'),
     path = require('path'),
     safe = require('safetydance'),
     shell = require('../shell.js');
@@ -78,6 +79,6 @@ describe('shell', function () {
 
     it('exec times out properly', async function () {
         const [error] = await safe(shell.promises.exec('sleeping', 'sleep 20', { timeout: 1000 }));
-        expect(error.signal).to.be('SIGTERM'); // somtimes code is ETIMEOUT
+        expect(error.reason).to.be(BoxError.SHELL_ERROR);
     });
 });