Add a better test for the extraCliArgs option

This uses testdouble.js to spy on the `ember` call that is done by `emberNew`.

tests/acceptance/helpers-test.js

@@ -33,14 +33,6 @@ describe('Acceptance: helpers', function () {
           expect(fs.existsSync(addonPath)).to.equal(true);
         });
     });
-
-    it('emberNew - extraCliArgs', () => {
-      return emberNew({ extraCliArgs: ['--typescript', '--no-welcome']})
-        .then(() => {
-          const appPath = path.resolve(process.cwd(), 'app');
-          expect(fs.existsSync(appPath)).to.equal(true);
-        });
-    });
   });
 
   describe('emberGenerateDestroy', () => {

tests/unit/ember-new-test.js

@@ -0,0 +1,38 @@
+'use strict';
+const setupTestHooks = require('../../lib/helpers/setup');
+
+const chai = require('../../chai');
+const expect = chai.expect;
+const td = require('testdouble')
+
+let ember;
+
+describe('Unit: emberNew', function () {
+  setupTestHooks(this);
+
+  afterEach(() => {
+    td.reset();
+    ember = undefined;
+  });
+
+  it('emberNew - extraCliArgs', () => {
+    const originalEmber = require('../../lib/helpers/ember.js');
+    ember = td.replace('../../lib/helpers/ember.js');
+
+    // "spy" on the original ember function.
+    // testdouble.js doesn't have built-in support for this because it considers it a bad practise:
+    // https://github.com/testdouble/testdouble.js/issues/512#issuecomment-1527511338
+    td.when(ember(td.matchers.contains('--typescript', '--no-welcome'), td.matchers.anything())).thenDo((...args) => {
+      return originalEmber(...args);
+    });
+
+    // We require emberNew here so the testdouble dependency replacement works
+    const emberNew = require('../../lib/ember-new');
+
+    return emberNew({ extraCliArgs: ['--typescript', '--no-welcome'] })
+      .then(() => {
+        // If we get here that means our testdouble matcher worked and things were called as expected.
+        expect(true).to.be.true;
+      });
+  });
+});