Make the multi-global tests more robust.

tests/wpt/web-platform-tests/fetch/api/request/multi-globals/incumbent/incumbent.html

@@ -6,9 +6,8 @@
 <script>
 'use strict';
 
-const current = document.querySelector('#c').contentWindow;
-
 window.createRequest = (...args) => {
+    const current = document.querySelector('#c').contentWindow;
     return new current.Request(...args);
 };
 

tests/wpt/web-platform-tests/fetch/api/request/multi-globals/url-parsing.html

@@ -18,7 +18,7 @@
 
 promise_test(() => {
     return loadPromise.then(() => {
-        const req = frames[0].createRequest("url");
+        const req = document.querySelector('iframe').contentWindow.createRequest("url");
 
         assert_equals(req.url, new URL("current/url", location.href).href);
     });

tests/wpt/web-platform-tests/fetch/api/response/multi-globals/incumbent/incumbent.html

@@ -7,10 +7,9 @@
 <script>
 'use strict';
 
-const current = document.querySelector('#c').contentWindow;
-const relevant = document.querySelector('#r').contentWindow;
-
 window.createRedirectResponse = (...args) => {
+    const current = document.querySelector('#c').contentWindow;
+    const relevant = document.querySelector('#r').contentWindow;
     return current.Response.redirect.call(relevant.Response, ...args);
 };
 

tests/wpt/web-platform-tests/fetch/api/response/multi-globals/url-parsing.html

@@ -18,7 +18,7 @@
 
 promise_test(() => {
     return loadPromise.then(() => {
-        const res = frames[0].createRedirectResponse("url");
+        const res = document.querySelector('iframe').contentWindow.createRedirectResponse("url");
 
         assert_equals(res.headers.get("Location"), new URL("current/url", location.href).href);
     });