Add test-helper generator.

Author: chriskrycho

blueprints/test-helper/files/tests/helpers/__name__.ts

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+import { registerAsyncHelper } from '@ember/test';
+
+export default registerAsyncHelper('<%= camelizedModuleName %>', function(app) {
+
+});

blueprints/test-helper/index.js

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+'use strict';
+
+module.exports = {
+  description: 'Generates a test helper.'
+};

node-tests/blueprints/test-helper-test.js

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+'use strict';
+
+const blueprintHelpers = require('ember-cli-blueprint-test-helpers/helpers');
+const setupTestHooks = blueprintHelpers.setupTestHooks;
+const emberNew = blueprintHelpers.emberNew;
+const emberGenerateDestroy = blueprintHelpers.emberGenerateDestroy;
+
+const chai = require('ember-cli-blueprint-test-helpers/chai');
+const expect = chai.expect;
+
+describe('Blueprint: test-helper', function() {
+  setupTestHooks(this);
+
+  describe('in app', function() {
+    beforeEach(function() {
+      return emberNew();
+    });
+
+    it('test-helper foo', function() {
+      return emberGenerateDestroy(['test-helper', 'foo'], _file => {
+        expect(_file('tests/helpers/foo.ts'))
+          .to.contain("import { registerAsyncHelper } from '@ember/test';")
+          .to.contain("export default registerAsyncHelper('foo', function(app) {\n\n}");
+      });
+    });
+  });
+});