test_runner: fix mock timer promisified setInterval return value

promisified setInterval can be passed a value that is returned by the
async iterable. The mocked promisified setInterval used this value for
its own purposes. This brings the mocked version in line with the
original.

Fixes: #50307

Author: mika-fischer

lib/internal/test_runner/mock/mock_timers.js

@@ -119,7 +119,7 @@ class MockTimers {
     this.#executionQueue.removeAt(position);
   }
 
-  async * #setIntervalPromisified(interval, startTime, options) {
+  async * #setIntervalPromisified(interval, result, options) {
     const context = this;
     const emitter = new EventEmitter();
     if (options?.signal) {
@@ -143,8 +143,7 @@ class MockTimers {
 
     const eventIt = EventEmitter.on(emitter, 'data');
     const callback = () => {
-      startTime += interval;
-      emitter.emit('data', startTime);
+      emitter.emit('data', result);
     };
 
     const timerId = this.#createTimer(true, callback, interval, options);