content: add tests for unimplemented code spans

Author: rajveermalviya

test/model/content_test.dart

@@ -293,6 +293,24 @@ void main() {
       QuotationNode([ParagraphNode(links: null, nodes: [TextNode('words')])]),
     ]);
 
+  testParse('parse code blocks, unknown span type',
+    '<div class="codehilite" data-code-language="Dart"><pre>'
+        '<span></span><code><span class="unknown">class</span>'
+        '\n</code></pre></div>', [
+          blockUnimplemented('<div class="codehilite" data-code-language="Dart"><pre>'
+            '<span></span><code><span class="unknown">class</span>'
+            '\n</code></pre></div>'),
+    ]);
+
+  testParse('parse code blocks, unsupported multiple span classes',
+    '<div class="codehilite" data-code-language="Dart"><pre>'
+        '<span></span><code><span class="class1 class2">class</span>'
+        '\n</code></pre></div>', [
+          blockUnimplemented('<div class="codehilite" data-code-language="Dart"><pre>'
+            '<span></span><code><span class="class1 class2">class</span>'
+            '\n</code></pre></div>'),
+    ]);
+
   testParse('parse code blocks, no language',
     // "```\nverb\natim\n```"
     '<div class="codehilite"><pre><span></span><code>verb\natim\n</code></pre></div>', const [