test(docs-infra): fix test for preview server's `BuildCleaner` completing prematurely (#25671)

PR Close #25671

aio/aio-builds-setup/dockerbuild/scripts-js/test/clean-up/build-cleaner.spec.ts

@@ -88,9 +88,12 @@ describe('BuildCleaner', () => {
     });
 
 
-    it('should return a promise', () => {
+    it('should return a promise', async () => {
       const promise = cleaner.cleanUp();
       expect(promise).toEqual(jasmine.any(Promise));
+
+      // Do not complete the test and release the spies synchronously, to avoid running the actual implementations.
+      await promise;
     });