MDL-52724 editor_atto: Scrub atto html for invalid ol, ul, and li tags

Try to correct broken ul/ol/li tags, as they have an outsized impact
on course layout. Uses basic regex and loops to track open and closed
tags.

Also adds a deep clean option to the HTML cleaner, that runs less
frequent, more intensive cleanings. This is because normally _cleanHTML
gets called after each keystroke, which could cause problems with
large content on weak systems.

Behat changes are a fix for setting multiline strings in Atto, and
creating a multiline match step.

lib/behat/form_field/behat_form_editor.php

@@ -51,6 +51,8 @@ public function set_value($value) {
         $editorid = $this->field->getAttribute('id');
         if ($this->running_javascript()) {
             $value = addslashes($value);
+            // This will be transported in JSON, which doesn't allow newlines in strings, so we must escape them.
+            $value = str_replace("\n", "\\n", $value);
             $js = '
 (function() {
     var editor = Y.one(document.getElementById("'.$editorid.'editable"));

lib/editor/atto/tests/behat/clean.feature

@@ -0,0 +1,169 @@
+@editor @editor_atto @atto @editor_moodleform
+Feature: Atto HTML cleanup.
+  In order to test html cleaning functionality, I write in a HTML atto text field.
+
+  @javascript
+  Scenario: Extra UL close and orphan LI items
+    Given I log in as "admin"
+    When I open my profile in edit mode
+    And I click on "Show more buttons" "button"
+    And I click on "HTML" "button"
+    And I set the field "Description" to multiline:
+    """
+        <li>A</li>
+        <li>B</li>
+    </ol>
+    <ul>
+        <li>C</li>
+    </ul></ul>
+    <li class="someclass ul UL">D</li>
+    <li>E</li>
+    """
+    And I click on "HTML" "button"
+    Then the field "Description" matches multiline:
+    """
+        <ol><li>A</li>
+        <li>B</li>
+    </ol>
+    <ul>
+        <li>C</li>
+    </ul>
+    <ul><li class="someclass ul UL">D</li>
+    <li>E</li></ul>
+    """
+
+  @javascript
+  Scenario: Missing LI close tags, extra closing OL, missing closing UL tag
+    Given I log in as "admin"
+    When I open my profile in edit mode
+    And I click on "Show more buttons" "button"
+    And I click on "HTML" "button"
+    And I set the field "Description" to multiline:
+    """
+    <div class="ol"><ol>
+        <li>A</li>
+        <li>B
+    </ol></div>
+    <ul>
+        <li>C
+        <li>D</li>
+    </ol>
+    """
+    And I click on "HTML" "button"
+    Then the field "Description" matches multiline:
+    """
+    <div class="ol"><ol>
+        <li>A</li>
+        <li>B
+    </li></ol></div>
+    <ul>
+        <li>C
+        </li><li>D</li></ul>
+
+    """
+
+  @javascript
+  Scenario: Missing beginning OL tag, empty LI close tag
+    Given I log in as "admin"
+    When I open my profile in edit mode
+    And I click on "Show more buttons" "button"
+    And I click on "HTML" "button"
+    And I set the field "Description" to multiline:
+    """
+    <p>Before</p>
+        <li>A</li></li>
+        <li>B</li>
+    </ol>
+    <p>After</p>
+    <ul data-info="UL ul OL ol">
+        <ul>
+            C</li>
+            <li>D</li>
+            <li>E
+        </ul>
+    </ul><ul>
+    <p>After 2</p>
+    """
+    And I click on "HTML" "button"
+    Then the field "Description" matches multiline:
+    """
+    <p>Before</p>
+        <ol><li>A</li>
+        <li>B</li>
+    </ol>
+    <p>After</p>
+    <ul data-info="UL ul OL ol">
+        <ul><li>
+            C</li>
+            <li>D</li>
+            <li>E
+        </li></ul>
+    </ul>
+    <p>After 2</p>
+    """
+
+  @javascript
+  Scenario: Random close LI tag, extra LI open tag, missing OL tag
+    Given I log in as "admin"
+    When I open my profile in edit mode
+    And I click on "Show more buttons" "button"
+    And I click on "HTML" "button"
+    And I set the field "Description" to multiline:
+    """
+    <p>Before</p></li><ul>
+    <ul>
+        <li>A</li>
+        B</li>
+        <li>C</li>
+    <ol>
+        <li>D</li>
+        <li>E
+    <p>After</p>
+    """
+    And I click on "HTML" "button"
+    Then the field "Description" matches multiline:
+    """
+    <p>Before</p>
+    <ul>
+        <li>A</li><li>
+        B</li>
+        <li>C</li></ul>
+    <ol>
+        <li>D</li></ol>
+        E
+    <p>After</p>
+    """
+
+  @javascript
+  Scenario: Missing opening LI tags, missing closing UL tag
+    Given I log in as "admin"
+    When I open my profile in edit mode
+    And I click on "Show more buttons" "button"
+    And I click on "HTML" "button"
+    And I set the field "Description" to multiline:
+    """
+    <li>Before</li>
+    <ul>
+        <li>A</li>
+        B</li>
+        <ol>
+            1</li>
+        </ol>
+        <li>C
+        <li>D</li>
+    <p>After</p>
+    """
+    And I click on "HTML" "button"
+    Then the field "Description" matches multiline:
+    """
+    <ul><li>Before</li></ul>
+    <ul>
+        <li>A</li><li>
+        B</li>
+        <ol><li>
+            1</li>
+        </ol>
+        <li>C
+        </li><li>D</li></ul>
+    <p>After</p>
+    """