build: fix failing test (#33212)

Fixes a compilation error in the Aria menu tests.

Author: crisbeto

src/aria/menu/menu.spec.ts

@@ -1,4 +1,4 @@
-import {Component, DebugElement} from '@angular/core';
+import {Component, DebugElement, signal} from '@angular/core';
 import {ComponentFixture, TestBed} from '@angular/core/testing';
 import {By} from '@angular/platform-browser';
 import {provideFakeDirectionality} from '@angular/cdk/testing/private';