stop using rfc268 adapter if not set

While it feels natural to auto-select an offcial RFC268 test mode,
if no adapter was specified; it adds some extra internal complexity,
because of eager import of the `@ember/test-helpers`.
In theory, it's possible to not have `@ember/test-helpers` installed,
which is demonstrated in "with-ember-test-helpers" ember-try scenario.

To solve this we could use `window.require(`, to lazily import needed helpers,
only in case if `@ember/test-helpers` installed. But this aproach may leads
to some issues in Node.js envs, since `window.require` is not a thibng there.
Also I'm not sure if `window.require(` is supposed to work fine with Emberoider.

So, we go an explicit way for now, and require to import,
and set a desired adapter on the test suite side manually.

addon-test-support/adapters/index.js

@@ -1,5 +1,4 @@
 import Adapter from "../adapter";
-import RFC268Adapter from "./rfc268";
 
 let _adapter;
 
@@ -8,7 +7,9 @@ let _adapter;
  */
 export function getAdapter() {
   if (!_adapter) {
-    return new RFC268Adapter();
+     throw new Error(`Adapter is required.
+
+Please use \`setAdapter(\`, to instruct "ember-cli-page-object" about the adapter you want to use.`);
   }
 
   return _adapter;

tests/acceptance/rfc268-actions-test.js

@@ -12,13 +12,11 @@ import {
 } from 'ember-cli-page-object';
 import { alias } from 'ember-cli-page-object/macros';
 
+import { setupApplicationTest } from 'dummy/tests/helpers'
+
 if (require.has('@ember/test-helpers')) {
   const { settled, waitUntil } = require('@ember/test-helpers');
 
-  // intentionally not using our local extension in order to make
-  // sure, RFC268 works by default, w/o Adapter being set.
-  const { setupApplicationTest } = require('ember-qunit');
-
   module('Acceptance | actions [rfc268]', function(hooks) {
     setupApplicationTest(hooks);
 

tests/acceptance/rfc268-test.js

@@ -1,6 +1,5 @@
 import { module, test } from 'qunit';
 import require from 'require';
-import Rfc268Adapter from 'ember-cli-page-object/adapters/rfc268';
 import { setAdapter } from 'ember-cli-page-object/adapters';
 import { create, value, visitable } from 'ember-cli-page-object';
 
@@ -9,6 +8,8 @@ if (require.has('@ember/test-helpers')) {
   // sure, RFC268 works by default, w/o Adapter being set.
   const { setupApplicationTest } = require('ember-qunit');
 
+  const Rfc268Adapter = require('ember-cli-page-object/adapters/rfc268').default;
+
   module('Acceptance | rfc268', function(hooks) {
     setupApplicationTest(hooks);