test(material/menu): referring to wrong overlay element (#23186)

Attempts to fix the recent flakiness in menu tests. It seems like we were destroying and recreating an overlay container at one point, but we were referring to the element of the previous.

Author: crisbeto

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

@@ -1244,7 +1244,7 @@ describe('MDC-based MatMenu', () => {
       TestBed.resetTestingModule();
 
       const newFixture = createComponent(SimpleMenu, [], [FakeIcon]);
-
+      overlayContainerElement = TestBed.inject(OverlayContainer).getContainerElement();
       newFixture.detectChanges();
       newFixture.componentInstance.trigger.openMenu();
       newFixture.detectChanges();

src/material/menu/menu.spec.ts

@@ -1241,7 +1241,7 @@ describe('MatMenu', () => {
       TestBed.resetTestingModule();
 
       const newFixture = createComponent(SimpleMenu, [], [FakeIcon]);
-
+      overlayContainerElement = TestBed.inject(OverlayContainer).getContainerElement();
       newFixture.detectChanges();
       newFixture.componentInstance.trigger.openMenu();
       newFixture.detectChanges();