test: menu tests flaky due to missing animation flush (#17935)

Follow-up to: 1255139

Two recently added tests for the menu/mdc-menu and the MDC-one from the
initial commit seem to miss a `tick` to flush the open animation. Without
the flush, the `FakeAsyncTestZone` will report a pending timer on test completion.

src/material-experimental/mdc-menu/menu.spec.ts

@@ -854,6 +854,7 @@ describe('MDC-based MatMenu', () => {
     fixture.detectChanges();
     fixture.componentInstance.trigger.openMenu();
     fixture.detectChanges();
+    tick(500);
 
     expect(document.activeElement)
         .toBe(overlayContainerElement.querySelector('.mat-mdc-menu-panel'));
@@ -865,6 +866,7 @@ describe('MDC-based MatMenu', () => {
     fixture.detectChanges();
     fixture.componentInstance.trigger.openMenu();
     fixture.detectChanges();
+    tick(500);
 
     expect(document.activeElement)
         .toBe(overlayContainerElement.querySelector('.mat-mdc-menu-panel'));

src/material/menu/menu.spec.ts

@@ -807,6 +807,7 @@ describe('MatMenu', () => {
     fixture.detectChanges();
     fixture.componentInstance.trigger.openMenu();
     fixture.detectChanges();
+    tick(500);
 
     expect(document.activeElement).toBe(overlayContainerElement.querySelector('.mat-menu-panel'));
   }));