WebKit export of https://bugs.webkit.org/show_bug.cgi?id=220541 (#27144)

Patch was r+ed downstream

beacon/beacon-common.sub.js

@@ -98,8 +98,7 @@ function CreateFormDataFromPayload(payload) {
     return formData;
 }
 
-// Schedules async_test's for each of the test cases, treating them as a single session,
-// and wires up the continueAfterSendingBeacon() and waitForResults() calls.
+// Schedules promise_test's for each of the test cases.
 // Parameters:
 //     testCases: An array of test cases.
 //     suffix [optional]: A string used for the suffix for each test case name.
@@ -108,10 +107,10 @@ function CreateFormDataFromPayload(payload) {
 function runTests(testCases, suffix = '', buildUrl = self.buildUrl, sendData = self.sendData) {
     for (const testCase of testCases) {
         const id = token();
-        async_test((test) => {
+        promise_test((test) => {
             const url = buildUrl(id);
             assert_true(sendData(url, testCase.data), 'sendBeacon should succeed');
-            waitForResult(id).then(() => test.done(), test.step_func((e) => {throw e;}));
+            return waitForResult(id);
         }, `Verify 'navigator.sendbeacon()' successfully sends for variant: ${testCase.name}${suffix}`);
     };
 }