Add tentative WPT of handling line breaks in inline editing host

Browsers shouldn't insert `<div>` and `<p>` elements into inline elements such
as `<span>` even when it's `display` value creates a block format context since
it's invalid structure from point of view of HTML.  However, some browsers do
it.  Therefore, this test should make them fail.

The reason why this patch touches `editor-test-utils.js` is, it scans the
following `<script>` element's text with a bug.  So the change makes it does
not scan outside the editing host.

Differential Revision: https://phabricator.services.mozilla.com/D150991

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1777903
gecko-commit: e3fd1571e2b9e6968d5e7b8063700afc469c4186
gecko-reviewers: m_kato

editing/data/insertparagraph.js

@@ -1930,18 +1930,8 @@ var browserTests = [
      "<div><br></div><div>  abc</div>"],
     [true,true],
     {"insertparagraph":[false,false,"",false,false,""]}],
-["<div contenteditable=false><span contenteditable>ab[]cd</span></div>",
-    [["defaultparagraphseparator","div"],["insertparagraph",""]],
-    ["<div contenteditable=\"false\"><span contenteditable=\"\">ab<br>cd</span></div>",
-     "<div contenteditable=\"false\"><span contenteditable=\"\">ab<br>cd<br></span></div>"],
-    [true,true],
-    {"insertparagraph":[false,false,"",false,false,""]}],
-["<p contenteditable=false><span contenteditable>ab[]cd</span></p>",
-    [["defaultparagraphseparator","div"],["insertparagraph",""]],
-    ["<p contenteditable=\"false\"><span contenteditable=\"\">ab<br>cd</span></p>",
-     "<p contenteditable=\"false\"><span contenteditable=\"\">ab<br>cd<br></span></p>"],
-    [true,true],
-    {"insertparagraph":[false,false,"",false,false,""]}],
+
+// TODO: Move the following tests into insertparagraph-or-insertlinebreak-in-inline-editing-host.tentative.html
 ["<h1 contenteditable=false><span contenteditable>ab[]cd</span></h1>",
     [["defaultparagraphseparator","div"],["insertparagraph",""]],
     ["<h1 contenteditable=\"false\"><span contenteditable=\"\">ab<br>cd</span></h1>",

editing/include/editor-test-utils.js

@@ -123,6 +123,9 @@ class EditorTestUtils {
         if (node.hasChildNodes()) {
           return inclusiveDeepestFirstChildNode(node);
         }
+        if (node === this.editingHost) {
+          return null;
+        }
         if (node.nextSibling) {
           return inclusiveDeepestFirstChildNode(node.nextSibling);
         }