Tweak HTMLOptionsCollection.length limit.

As per the discussion in whatwg/html#8347

Differential Revision: https://phabricator.services.mozilla.com/D160544

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1797792
gecko-commit: bdd3a81d328efa4627647bcc54e8b09878014c92
gecko-reviewers: smaug

html/select/options-length-too-large.html

@@ -23,13 +23,21 @@
     });
 
     test(function() {
-        mySelect.options.length = 10001;
+        mySelect.options.length = 100001;
         assert_equals(mySelect.options.length, 3, "Length of <select> should remain unchanged");
     });
 
     test(function() {
-        mySelect.options.length = 10000;
-        assert_equals(mySelect.options.length, 10000, "Length of <select> should be 10,000");
+        mySelect.options.length = 100000;
+        assert_equals(mySelect.options.length, 100000, "Length of <select> should be 10,0000");
+    });
+
+    test(function() {
+        mySelect.appendChild(new Option());
+        mySelect.appendChild(new Option());
+        assert_equals(mySelect.options.length, 100002, "Manual expansion still works");
+        mySelect.options.length = 100001;
+        assert_equals(mySelect.options.length, 100001, "Truncation works if over the limit");
     });
     </script>
   </body>