Added basic system tests for markdown editor, Added extra test helpers

Added test helpers for checking if an element exists / does not exist on a page.
Also fixed markdown editor bugs found while creating tests.

tests/Entity/MarkdownTest.php

@@ -0,0 +1,51 @@
+<?php
+
+
+class MarkdownTest extends TestCase
+{
+    protected $page;
+
+    public function setUp()
+    {
+        parent::setUp();
+        $this->page = \BookStack\Page::first();
+    }
+
+    protected function setMarkdownEditor()
+    {
+        $this->setSettings(['app-editor' => 'markdown']);
+    }
+
+    public function test_default_editor_is_wysiwyg()
+    {
+        $this->assertEquals(setting('app-editor'), 'wysiwyg');
+        $this->asAdmin()->visit($this->page->getUrl() . '/edit')
+            ->pageHasElement('#html-editor');
+    }
+    
+    public function test_markdown_setting_shows_markdown_editor()
+    {
+        $this->setMarkdownEditor();
+        $this->asAdmin()->visit($this->page->getUrl() . '/edit')
+            ->pageNotHasElement('#html-editor')
+            ->pageHasElement('#markdown-editor');
+    }
+
+    public function test_markdown_content_given_to_editor()
+    {
+        $this->setMarkdownEditor();
+        $mdContent = '# hello. This is a test';
+        $this->page->markdown = $mdContent;
+        $this->page->save();
+        $this->asAdmin()->visit($this->page->getUrl() . '/edit')
+            ->seeInField('markdown', $mdContent);
+    }
+
+    public function test_html_content_given_to_editor_if_no_markdown()
+    {
+        $this->setMarkdownEditor();
+        $this->asAdmin()->visit($this->page->getUrl() . '/edit')
+            ->seeInField('markdown', $this->page->html);
+    }
+
+}