Change namespace for Trusted-Types tests for setAttributeNS to null

See
<w3c/trusted-types#418 (comment)>
and
<w3c/trusted-types#418 (comment)>.

Preparation to fix <w3c/trusted-types#425>.

Author: Mirko Brodesser

trusted-types/block-string-assignment-to-Element-setAttributeNS.html

@@ -21,7 +21,7 @@
       assert_element_accepts_trusted_script_url_set_ns(window, '2', t, 'a', 'b', RESULTS.SCRIPTURL);
     }, "Element.setAttributeNS assigned via policy (successful ScriptURL transformation)");
 
-    // Unknown, namespaced attributes should not be TT checked:
+    // Unknown attributes should not be TT checked:
     test(t => {
       assert_element_accepts_non_trusted_type_set_ns('a', 'b', 'A string', 'A string');
     }, "Element.setAttributeNS accepts untrusted string for non-specced accessor");

trusted-types/support/helper.sub.js

@@ -125,7 +125,7 @@ function assert_element_accepts_non_trusted_type_explicit_set(tag, attribute, va
   assert_equals(elem.getAttribute(attribute), expected);
 }
 
-let namespace = 'http://www.w3.org/1999/xhtml';
+let namespace = null;
 function assert_element_accepts_trusted_html_set_ns(win, c, t, tag, attribute, expected) {
   let p = createHTML_policy(win, c);
   let html = p.createHTML(INPUTS.HTML);