Complete modal dialog tests.

Problem/Motivation

Follow up for #1667742: Add abstracted dialog to core (resolves accessibility bug)
This is from the patch committed in #1667742-110: Add abstracted dialog to core (resolves accessibility bug)

+++ b/core/modules/system/lib/Drupal/system/Tests/Ajax/DialogTest.phpundefined
@@ -0,0 +1,32 @@
+  /**
+   * Ensure elements with #ajax['dialog'] render correctly.
+   */
+  function testDialog() {
+    // Ensure the elements render without notices or exceptions.
+    $this->drupalGet('ajax-test/dialog');
+
+    // @todo What else should we assert?
+  }
+
+}

So currently no way to test Dialog behavior applied to a button or link.

Proposed resolution

Complete the testDialog function in Drupal\system\Tests\Ajax. and remove // @todo What else should we assert?.

Remaining tasks

• Complete the testDialog function.
• #1842036: [META] Convert all confirm forms to use modal dialog is postpone on this.