Use proper command in executeCli-test

In order to test the output of the cli command, we need to stop using
the js module directly. Instead, we can use the built cli.js file. This
is the same file that users end up invoking when running `happo run`.

test/executeCli-test.js

@@ -1,12 +1,8 @@
-import commander from 'commander';
+import { execSync } from 'child_process';
 
-import executeCli from '../src/executeCli';
-
-beforeEach(() => {
-  commander.help = jest.fn();
-});
+const happoCommand = require.resolve('../build/cli');
 
 it('displays help when no command is given', () => {
-  executeCli(['node', 'happo']);
-  expect(commander.help.mock.calls.length).toBe(1);
+  const result = execSync(happoCommand, { encoding: 'utf-8' });
+  expect(result).toMatch(/^Usage: /);
 });