Move promise_test and assert_promise_rejects out of worker-test-harness.

LayoutTests/http/tests/serviceworker/resources/request-worker.js

-importScripts('worker-test-harness.js');
+importScripts('worker-testharness.js');
+importScripts('../../resources/testharness-helpers.js');
 importScripts('test-helpers.js');
 
 var URL = 'https://www.example.com/test.html';
 
 function size(headers) {
   var count = 0;
   for (var header of headers) {
     ++count;
   }
   return count;
@@ skipping 426 matching lines @@
             result, expected_body,
             'Creating a Request with FormData body should success.');
         })
       .then(function() {
           t.done();
         })
       .catch(unreached_rejection(t));
     assert_true(request.bodyUsed,
                 'bodyUsed must be true after calling text()');
   }, 'Request body test in ServiceWorkerGlobalScope');