LibWeb: Don't process frameset token twice in "in body" insertion mode

We were neglecting to return after handling the `frameset` start tag,
which caused us to process it twice, once properly and once generically.

54 new passes in WPT/html/syntax/parsing/ :^)

Libraries/LibWeb/HTML/Parser/HTMLParser.cpp

@@ -1897,6 +1897,8 @@ void HTMLParser::handle_in_body(HTMLToken& token)
 
         // 4. Switch the insertion mode to "in frameset".
         m_insertion_mode = InsertionMode::InFrameset;
+
+        return;
     }
 
     // -> An end-of-file token