refactor: move spec helpers to spec/lib (#37010)

Co-authored-by: Milan Burda <miburda@microsoft.com>

spec/lib/events-helpers.ts

@@ -0,0 +1,55 @@
+/**
+ * @fileoverview A set of helper functions to make it easier to work
+ * with events in async/await manner.
+ */
+
+/**
+ * @param {!EventTarget} target
+ * @param {string} eventName
+ * @return {!Promise<!Event>}
+ */
+export const waitForEvent = (target: EventTarget, eventName: string) => {
+  return new Promise(resolve => {
+    target.addEventListener(eventName, resolve, { once: true });
+  });
+};
+
+/**
+ * @param {!EventEmitter} emitter
+ * @param {string} eventName
+ * @return {!Promise<!Array>} With Event as the first item.
+ */
+export const emittedOnce = (emitter: NodeJS.EventEmitter, eventName: string, trigger?: () => void) => {
+  return emittedNTimes(emitter, eventName, 1, trigger).then(([result]) => result);
+};
+
+export const emittedNTimes = async (emitter: NodeJS.EventEmitter, eventName: string, times: number, trigger?: () => void) => {
+  const events: any[][] = [];
+  const p = new Promise<any[][]>(resolve => {
+    const handler = (...args: any[]) => {
+      events.push(args);
+      if (events.length === times) {
+        emitter.removeListener(eventName, handler);
+        resolve(events);
+      }
+    };
+    emitter.on(eventName, handler);
+  });
+  if (trigger) {
+    await Promise.resolve(trigger());
+  }
+  return p;
+};
+
+export const emittedUntil = async (emitter: NodeJS.EventEmitter, eventName: string, untilFn: Function) => {
+  const p = new Promise<any[]>(resolve => {
+    const handler = (...args: any[]) => {
+      if (untilFn(...args)) {
+        emitter.removeListener(eventName, handler);
+        resolve(args);
+      }
+    };
+    emitter.on(eventName, handler);
+  });
+  return p;
+};