Add a test to validate the structure of the heading nav

This checks the entire structure of the heading nav for this test. It
currently is not handling nesting properly.

Having a large inline string like this may not be the easiest to
maintain or be able to see the structure. I don't see a way to format it
in goml.

Author: ehuss

tests/gui/heading-nav-collapsed.goml

@@ -10,6 +10,8 @@ assert-text: (".current-header", "Heading 1")
 assert-attribute: ("li:has(> span > a[href='#heading-12'])", {"class": "header-item"})
 assert-attribute: ("li:has(> span > a[href='#heading-21'])", {"class": "header-item"})
 
+assert-property: ("div.on-this-page", {"innerHTML": '<ol class="section"><li class="header-item expanded"><span class="chapter-link-wrapper"><a href="#heading-1" class="header-in-summary current-header">Heading 1</a></span><ol class="section"><li class="header-item expanded"><span class="chapter-link-wrapper"><a href="#heading-11" class="header-in-summary">Heading 1.1</a></span></li><li class="header-item"><span class="chapter-link-wrapper"><a href="#heading-12" class="header-in-summary">Heading 1.2</a><a class="chapter-fold-toggle header-toggle"><div>❱</div></a></span><ol class="section"><li class="header-item expanded"><span class="chapter-link-wrapper"><a href="#heading-121" class="header-in-summary">Heading 1.2.1</a></span></li><li class="header-item expanded"><span class="chapter-link-wrapper"><a href="#heading-122" class="header-in-summary">Heading 1.2.2</a></span></li></ol></li></ol></li><li class="header-item expanded"><span class="chapter-link-wrapper"><a href="#heading-13" class="header-in-summary">Heading 1.3</a></span></li><li class="header-item expanded"><span class="chapter-link-wrapper"><a href="#heading-2" class="header-in-summary">Heading 2</a></span><ol class="section"><li class="header-item"><span class="chapter-link-wrapper"><a href="#heading-21" class="header-in-summary">Heading 2.1</a><a class="chapter-fold-toggle header-toggle"><div>❱</div></a></span><ol class="section"><li class="header-item"><span class="chapter-link-wrapper"><a href="#heading-211" class="header-in-summary">Heading 2.1.1</a><a class="chapter-fold-toggle header-toggle"><div>❱</div></a></span><ol class="section"><li class="header-item"><span class="chapter-link-wrapper"><a href="#heading-2111" class="header-in-summary">Heading 2.1.1.1</a><a class="chapter-fold-toggle header-toggle"><div>❱</div></a></span><ol class="section"><li class="header-item expanded"><span class="chapter-link-wrapper"><a href="#heading-21111" class="header-in-summary">Heading 2.1.1.1.1</a></span></li></ol></li></ol></li></ol></li></ol></li></ol>'})
+
 // Click 1.2, expands it.
 click: "a.header-in-summary[href='#heading-12']"
 assert-attribute: ("li:has(> span > a[href='#heading-12'])", {"class": "header-item expanded"})