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.

(cherry picked from commit 3ae6124)

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();